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Forensic Analysis: Weighing Bullet Lead Evidence (2004)

Chapter: 4. Interpretation

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Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

4
Interpretation

The primary objective of compositional analysis of bullet lead (CABL) is to produce evidence for use in court. Although the evidence is analyzed with scientific instrumentation and statistical methods, its presentation and use in court are subject to human interpretation and error. Attorneys, judges, juries, and even expert witnesses can easily and inadvertently misunderstand and misrepresent the analysis of the evidence and its importance. It is therefore essential to discuss whether and how the evidence can be used. It is first necessary to introduce the lead and bullet manufacturing processes so that the implications of bullet production for the legal system are fully understood. This chapter is split into two sections: “Significance of the Bullet Manufacturing Process” and “Compositional Analysis of Bullet Lead as Evidence in the Legal System.”

SIGNIFICANCE OF THE BULLET MANUFACTURING PROCESS

The following description of the processes leading to the production of loaded ammunition represents the bullet manufacturing practices currently in place at large-scale producers in the United States. (Processes used overseas are less well documented.) As shown in this chapter, the processes vary at numerous points, depending on such factors as the manufacturer, the caliber and style of bullet, the magnitude of a production run (which is often dictated by the demand for a particular caliber), and the size of the manufacturing facility. This

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

section details procedures that are believed to account for the manufacturing processes used for .22 caliber rimfire and other bullets by major producers in the United States. (This process is described because .22 caliber rimfire ammunition is one of the most popular ammunition rounds produced.) It has been estimated that 50–75 percent of all ammunition sold in the United States originates with U.S. manufacturers and that about 50 percent of ammunition used by the U.S. military (for example, 9-mm, 7.62-NATO, and 5.56-NATO ammunition) and more than 50 percent of non-U.S. issue military calibers (such as 7.62 × 39 <AK-47> and British .303 <Enfield>) are imported.1,2,3

GENERAL INFORMATION ON BULLETS

On the order of 85–118 million pounds of lead is used each year in the production of bullets4 in the United States.5,6 The exact number of each caliber and type of bullet (such as jacketed or hollow point) is not known, but some estimates of production volumes have been provided by the Sporting Arms and Ammunition Manufacturers’ Institute7 and are shown in Table 4.1. It is generally acknowledged that .22 caliber bullets are the dominant type sold. Table 4.2 provides some examples of typical bullet masses for various calibers. Using 70 grains (0.16 oz, 4.54 g) as an arbitrarily assumed average bullet mass allows the estimation that the 85–118 million pounds of bullet lead produces about 8.5–11.8 billion bullets per year in the United States.

OVERVIEW OF BULLET PRODUCTION

Figure 4.1 is a simplified flow chart for bullet production and approximate mass of material involved in each of the processed materials. Table 4.3 has been prepared from the general information given in Figure 4.1 to illustrate the approximate number of bullets associated with each of the manufacturing steps or

1  

Greenberg, R. R. March 3, 2003. Verbal communication to committee after visiting the SHOT Show February 13–16, 2003.

2  

Shotgun News Special Interest Publications, Peoria, IL May 20, 2003. A collection of firearms related advertisements for retailers and wholesalers.

3  

CABL also has value for the matching of foreign-produced bullet lead; this value varies according to the lead’s nation of origin and that nation’s lead recycling and manufacturing processes. The analysis of foreign-produced bullets is not discussed in detail in this report.

4  

The committee assumes these numbers include lead for shot as well as bullets.

5  

Biviano, M. B.; Sullivan, D. E.; Wagner, L. A. Total Materials Consumption: An Estimation Methodology and Example Using Lead—A Materials Flow Analysis. USGS Circular: 1183. April, 1999. <http://pubs.usgs.gov/circ/1999/c1183>.

6  

Smith, G. R. USGS Minerals Yearbook 2001: Lead. Reston, VA 2001. <http://minerals.er.usgs.gov/minerals/pubs/commodity/lead/leadmyb01.pdf>.

7  

Green, K. D. Introduction to the Bullet Manufacturing Process: Committee on Scientific Assessment of Bullet Lead Elemental Composition Comparison, Washington, DC February 3, 2003.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

TABLE 4.1 Annual Production of Ammunitions Produced in the United States

Ammunition Type

No. Rounds Produced per Year, billions

No. Boxes Produced per Year, millions

No. Units per Box

Shotgun shells (all gauges)

1.1

44

25

Rifle, center fire

0.25

12.5

20

Pistol and revolver, center fire

0.55

11

50

Rifle and pistol, rimfire

2

40

50

 

Source: See Footnote 7.

TABLE 4.2 Examples of Various Caliber and Style of Bullets and Estimated Bullet Mass

Caliber

Style

Total Mass of Projectile (Mass of Pb if Jacketed)

Grains

Ounces

Grams

.22 Long rifle

Round nose/Hollow point

40

0.0914

2.59

9 × 19 mm

Lead round nose

124

0.283

8.04

9 × 19 mm

Full metal jacket

124 (103.0)

0.283 (0.237)

8.04 (6.71)

.38 special

Lead round nose

150

0.343

9.72

44 Remington magnum

Lead truncated cone

240

0.549

15.6

5.56 × 45 mm

Full metal jacket

62 (31.6)

0.142 (0.0722)

4.02 (2.05)

5.56 × 45 mm

Full metal jacket

55 (46.1)

0.126 (0.105)

3.56 (2.99)

7.62 × 51 mm

Full metal jacket

145 (93.1)

0.331 (0.213)

9.40 (6.03)

products. Calculations assumed a mass of 40 grains (0.0914 oz, 2.59 g) for a .22 rimfire projectile. The number of projectiles is based on 100 percent yield. Since some material is not converted directly to the final bullets (for example, initial piece of extruded wire, weep from bullet presses), the actual number of projectiles produced will be lower.

In the United States, secondary smelters melt recycled lead (primarily from recycled lead-acid storage batteries) for bullet lead processing in large pots.8 The designation of primary smelter is reserved for manufacturing facilities that produce lead from ores. Such facilities are rarely associated directly with bullet production in the United States, but this is not the case in some foreign countries. Secondary smelting is reported to account for half the lead produced in the

8  

Smith, G. R. Lead Recycling in the United States in 1998. USGS Circular: 1196-F. 2002. <http://pubs.usgs.gov/circ/c1196f/>.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

FIGURE 4.1 Flow diagram of bullet materials, a general description of the many steps involved in bullet production.

TABLE 4.3 Approximate Masses and Numbers of Bullets Produced from “Single Unit” of Various Stages in Manufacturing Processa

Source of Material

Weight of Material (lbs)

Mass of Material (kg)

Yield (of .22 Caliber Bullets)

Melt pot

200,000

90,719

35,000,000

Melt pot

100,000

45,360

17,500,000

Sow

2,000

907

350,000

Billet

70–350

32–159

12,250–61,250

Pig/Ingot

60–125

27–57

10,500–21,875

aGreen, K. D. Introduction to the Bullet Manufacturing Process: Committee on Scientific Assessment of Bullet Lead Elemental Composition Comparison, Washington, DC February 3, 2003.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

United States. There are 50 plants, with capacities ranging from 1,000 to 120,000 tons/year.9

Refining of the melt to remove various elements present either as impurities or as previously added alloy elements can occur at the secondary smelter.10,11 After refinement, Sb, less frequently Sn, and sometimes both elements may be added to harden the bullet. Finally, the melt is poured into various smaller products, including billets, which are sent to the bullet manufacturer.

The bullet manufacturer may use the purchased billets directly for production, but it is not uncommon for bullet manufacturers to remelt the purchased lead and cast their own billets for production.12 The bullet manufacturer extrudes bullet wire from a solid billet; this results in one or more wires per billet, depending on whether the extruder die has one or more extrusion ports. Generally, a continuous wire is not produced from multiple billets due to the likelihood of discontinuity and the production of a flawed slug at the junction due to lead lamination. The size of the extruded wire is dictated by the caliber (diameter) of the bullet to be produced from that wire.

The bullet wire is then fed into a machine that cuts it to predetermined lengths to generate slugs of the approximate weight and dimensions of the final bullet. The slugs are collected in bins, whose size varies from plant to plant. In larger manufacturing facilities, several extruders may be operated in parallel in the production of slugs of a given caliber, and the slugs from the various extruders may be collected in the same bin. A given wire is converted to slugs of a given length and diameter.

The slugs are then pressed into the final shape of the bullet, a jacket is applied (if appropriate), and the bullets are again collected in bins.13 The bullets are seated into appropriately prepared cartridge cases (loaded with primer and powder) to form the loaded ammunition, which is either collected in bins or sent directly to machinery for packing in boxes. The boxes generally contain 20–50 rounds each, depending on the caliber and the products being offered by the company. A more specific example of the wire-to-ammunition production steps for .22 caliber rimfire bullet production is as follows:14

9  

U.S. Environmental Protection Agency. Compilation of Air Pollutant Emission Factors, AP-42, Fifth Edition, Volume I: Stationary Point and Area Scources, Secondary Lead Chapter 12 section 11. Research Triangle Park, NC, January 1995.

10  

Randich, E.; Duerfeldt, W.; McLendon, W.; and Tobin, W. Foren. Sci. Int. 2002, 127, 174–191.

11  

Frost, G. E. Ammunition Making, Chapter 3. National Rifle Association of America, Washington, DC 1990, 25–43.

12  

Green, K. D. Introduction to the Bullet Manufacturing Process: Committee on Scientific Assessment of Bullet Lead Elemental Composition Comparison, Washington, DC February 3, 2003.

13  

Bullet cores are extruded as wires of a slightly smaller diameter than for unjacketed bullets of the same caliber, are cut into slugs, and are swaged into thimble-like jackets. The production of bullet cores is otherwise identical to the production of bullets.

14  

Green, K. D. Introduction to the Bullet Manufacturing Process: Committee on Scientific Assessment of Bullet Lead Elemental Composition Comparison, Washington, DC February 3, 2003.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×
  • Pinch cut to partially perforate the wire.

  • Tumble the partially perforated wire to break it into slugs.

  • Swage press to final shape (three steps are needed).

  • Wash and rinse.

  • Flash plate with copper alloy (if high-velocity product is being made).

  • Lubricate.

  • Assemble into loaded ammunition.

  • Pack ammunition in boxes.

The boxes are then generally bundled into appropriately sized shipping quantities—such as cartons, crates, or pallets—and sent to jobbers, distributors, wholesalers, or large retailers. They then go to the retailer’s shelf for purchase by the consumer.

Reloaders, both commercial and private, are another source of loaded ammunition and are less directly connected to large-volume manufacturers.15 Using refurbished cases for reloading, reloaders make less-expensive ammunition. In some instances, reloaded bullets are made from melted scrap lead, such as discarded wheel-balancing weights that are remelted and poured into bullet molds.

DETAILS OF BULLET PRODUCTION

This section details the various stages leading to the production and distribution of boxes of loaded ammunition. Comments on the variations that are known to exist at various stages are given here, but their implications for the homogeneity of melts, billets, wires, and so on, are discussed in the section titled “Compositional Information.”

Sources and Use of Lead

With over 3.5 billion pounds of lead smelted each year in the Unites States, the 85–118 million pounds used in bullet manufacturing comprises about 2.5–3 percent of total lead use; lead-acid storage batteries probably represent the largest product.16,17 Secondary smelters that produce bullet lead are also gen-

15  

Commercial reloaders are often known as remanufacturers. The concentrations of elements in component bullets used by reloaders are similar to the concentrations in bullet lead used by major manufacturers. Component bullet unit sales are a small fraction (5–10 percent) of loaded ammunition sales, but can follow wider distribution channels because there are fewer shipping restrictions. Reloaded ammunition is not expected to comprise a large percentage of the ammunition involved in casework.

16  

Biviano, M. B.; Sullivan, D. E.; Wagner, L. A. Total Materials Consumption: An Estimation Methodology and Example Using Lead—A Materials Flow Analysis. USGS Circular: 1183. April, 1999. <http://pubs.usgs.gov/circ/1999/c1183>.

17  

Smith, G. R. USGS Minerals Yearbook 2001: Lead. Reston, VA 2001. <http://minerals.er.usgs.gov/minerals/pubs/commodity/lead/leadmyb01.pdf>.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

erally involved in the production of “battery lead.” Chemical compositional requirements for bullet lead are much less stringent (that is, they have less-restrictive tolerances) than are needed for battery lead. However, a hardened lead is generally needed for bullets.18,19 Hardening is typically accomplished by the addition of Sb to the melt. Sn can also be used, but it is more expensive. Other components of bullet lead are generally carried over from the lead source, and maximal tolerances in their concentrations are normally specified by the bullet manufacturer.

Bullets are reportedly produced mainly from recycled lead in the United States. Therefore, it is impossible to trace bullet lead back to the original source of the ore,20 and no detailed discussion will be presented here on the primary smelters and ore processing except to note that the ores are sulfides and contain small amounts of Cu, Fe, Zn, precious metals, and other trace and minor elements, such as As, Sb, and Bi. The primary smelting process involves removal of those elements by reduction and refining.

Secondary Lead Smelters

As noted previously, the dominant source of bullet lead is the electrode materials from recycled batteries. The melting process takes place in pots that may contain, for example, 50–350 tons of melt. The descriptions given below are typical; they might not be applicable to all smelters.

The first step in secondary lead refining is treatment of scrap to remove metallic and nonmetallic contaminants. That is done by mechanical breaking and crushing to separate extraneous contaminants and then “sweating” the separated lead scrap in a reverberatory furnace to isolate the lead from metals that have higher melting points. The next step is smelting in a blast furnace to make “hard” (high-Sb) lead or in a reverberatory furnace to make “semisoft” (3–4 percent Sb) lead. Refining is normally done in a batch process that takes a few hours to a few days in kettle-type furnaces that have production capacities of 25–150 tons/day.21 In the refining process, Cu, Sb, As, and Ni are the main elements removed. It is generally assumed that Sb is the element whose content is most critical because it determines the bullet hardness.22,23

18  

Randich, E.; Duerfeldt, W.; McLendon, W.; and Tobin, W. Foren. Sci. Int. 2002, 127, 174–191.

19  

Peters, C.; Havekost, D. G.; Koons, R. D. Crime Lab. Digest 1988, 15(2), 33–38.

20  

Smith, G. R. USGS Minerals Yearbook 2001: Lead. Reston, VA 2001. <http://minerals.er.usgs.gov/minerals/pubs/commodity/lead/leadmyb01.pdf>.

21  

U.S. Environmental Protection Agency. Compilation of Air Pollutant Emission Factors, AP-42, Fifth Edition, Volume I: Stationary Point and Area Sc=ources, Secondary Lead Chapter 12 section 11. Research Triangle Park, NC, January 1995.

22  

Randich, E.; Duerfeldt, W.; McLendon, W.; and Tobin, W. Foren. Sci. Int. 2002, 127, 174–191.

23  

Peters, C.; Havekost, D. G.; and Koons, R. D. Crime Lab. Digest 1988, 15(2), 33–38.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

TABLE 4.4 Example of Manufacturer’s Compositional Requirements for Pb to Be Used in .22 Long Rifle Projectilesa

Preferred Analysis

Weight Percent

Sb

0.85 ± 0.15 %

Maximal Impurities

Weight Percent

Al

0.001%

As

0.05–0.10%

Bi

0.05%

Cd

0.001%

Cu

0.03%

Ca

0.001%

Fe

0.001%

Ni

0.001%

Se

0.002%

Ag

0.01%

S

0.001%

Te

0.01%

Sn

0.15–0.2%

Zn

0.001%

Sow Size

Weight in Pounds

Maximum

2,200 lb

Minimum

1,500 lb

aPrengaman, R. D. Lead and Lead Refining: Committee on Scientific Assessment of Bullet Lead Elemental Composition Comparison, Washington, DC March 3, 2003.

In the production of bullet lead, the manufacturer generally has requirements for the concentrations of the final lead alloy.24 The elemental compositional requirements can vary with the bullet manufacturer. Depending on the element, either maximal allowable or ranges of concentrations may be specified. Table 4.4 shows an example of one manufacturer’s compositional requirements for lead to be used in .22 long rifle bullets. Some bullet producers use as-received billets from secondary smelters, and others conduct tertiary melting to make additional adjustments to the lead composition or to recycle scraps of lead produced during bullet production.

A secondary smelter may produce solid lead of various shapes, including ingots, pigs, and billets. An analysis certificate accompanies the product shipped to the bullet manufacturer; it uses a smelter-dependent format that contains various degrees of analytical detail. Spark-emission optical spectroscopy is the technique generally used for analysis of the alloy at the smelters.25 The technique

24  

Prengaman, R. D. Lead and Lead Refining: Committee on Scientific Assessment of Bullet Lead Elemental Composition Comparison, Washington, DC March 3, 2003.

25  

Prengaman, R. D. Lead and Lead Refining: Committee on Scientific Assessment of Bullet Lead Elemental Composition Comparison, Washington, DC March 3, 2003.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

generally produces precision on the order of ±10–20%; however, when the most stringent standardization procedures are implemented, precision may approach ±5 percent.26

There is no requirement by the bullet manufacturers that all lead ingots received from a smelter come from a single pour or melt. It is generally assumed that the composition of a given melt is constant and homogeneous from the beginning to the end of the pour if nothing is added to the pot during the pour.27 The assumption of homogeneity is based on the convective mixing in the vat and the relatively short pouring time. It should be noted that during a pour material may be added to the original melt, thus producing time-varying compositional changes. The additions may include bulk material (ingots, pigs, and so on), manufacturing scrap (pieces of bullet wire, scrap from bullet-forming operations, and the like), or molten lead introduced from a secondary vat. Examples of the time-dependent variation in composition can be seen in some of the data of Koons and Grant.28 In the case of at least one manufacturer, billets are not poured from a vat that has a constant composition; instead, while the vat is being poured, molten lead from another pot is continuously added to maintain the level of molten lead in the vat being poured. Thus, compositional changes can occur during casting. The data of Koons and Grant29 indicate that compositional change occurs over several 60 lb ingots that were being poured. For example, the concentration of Sn decreased by 60 percent (from 0.030 to 0.012 percent Sn) over a 30 minute period, the largest change of the data presented. Combining this information with the standard deviations for the analytical measurement (that is, < 0.001 percent Sn) it can be estimated that approximately 15 ingots (approximately 850 lbs of Pb) were poured before the average concentrations changed by one standard deviation. Thus, it can be reasonably assumed that the rate of compositional change—even when molten lead batches are mixed during a pour—from one poured ingot to the next poured ingot is much smaller than the measurement precision available. It also follows that any compositional change in the lead initially poured into an ingot (or billet) would be indistinguishable from the molten lead added to the mold to complete the pour of that ingot, as long as the casting of the ingot was completed in a single pour.

Randich et al.30 also showed occasional distinct concentration changes in some elements as samples were extracted from the beginning, middle, and end of the pour. Statistical analysis of the changes showed that there was no distinct time-dependent one-directional change (that is, always increasing or decreasing

26  

Mitteldorf, A. J. In Trace Analysis; Morrison, G. H., Ed.; John Wiley and Sons: New York, 1965, pp 193–243.

27  

Prengaman, R. D. Lead and Lead Refining: Committee on Scientific Assessment of Bullet Lead Elemental Composition Comparison, Washington, DC March 3, 2003.

28  

Koons, R. D. and Grant, D. M. J. Foren. Sci. 2002, 47, 950–958.

29  

Koons, R. D. and Grant, D. M. J. Foren. Sci. 2002, 47, 950–958.

30  

Randich, E.; Duerfeldt, W.; McLendon, W.; and Tobin, W. Foren. Sci. Int. 2002, 127, 174–191.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

as the pour proceeded), which would suggest for these data that lead of a different composition was being added during the pour, rather than that some chemical process occurred that depleted or enriched a given element as a function of time. The former possibility (the addition of lead during the pour) is supported by the data of Koons and Grant,31 who presented a more detailed analysis of billets resulting from pours. Koons and Grant used several of the same data sets as Randich et al.32

Billet Production

Billets weigh 70–350 lbs (32–159 kg), depending on the manufacturer and the size and type of extruder that is used in the production of bullet wire.33,34 In some instances, the secondary smelter is also a bullet manufacturer, and the billets produced are used on site in the production of wire, slugs, and so forth. In other instances, the lead ingots, pigs, or billets are shipped to bullet manufacturers, and the bullet manufacturers may use the billets directly in their extruders to produce wire. There are also instances in which the ingots or pigs obtained from the secondary smelters are remelted to pour new billets at the bullet manufacturing plant.

Various activities can occur during this tertiary melting that affect the final billet composition. For example, melted lead prior to casting in billets is typically “fluxed” to remove oxidized lead metal elements and other impurities. The fluxing agent can contain a number of different materials, and is often borate-based in commercial bullet manufacturing operations. Nitrogen gas is also a common fluxing agent. The flux entrains the impurities and floats them to the surface of the lead melt for removal.

Bullet Production

Billets are used without alteration (in their original, solid state) in the extruders to produce bullet wire. The mass of the wire is somewhat less than the mass of the billet, because the tail end of the billet cannot be forced through the extrusion die by the ram.35,36 The length of the wire is governed by the billet

31  

Koons, R. D. and Grant, D. M. J. Foren. Sci. 2002, 47, 950–958.

32  

Randich, E.; Duerfeldt, W.; McLendon, W.; and Tobin, W. Foren. Sci. Int. 2002, 127, 174–191.

33  

Green, K. D. Introduction to the Bullet Manufacturing Process: Committee on Scientific Assessment of Bullet Lead Elemental Composition Comparison, Washington, DC February 3, 2003.

34  

Prengaman, R. D. Lead and Lead Refining: Committee on Scientific Assessment of Bullet Lead Elemental Composition Comparison, Washington, DC March 3, 2003.

35  

Green, K. D. Introduction to the Bullet Manufacturing Process: Committee on Scientific Assessment of Bullet Lead Elemental Composition Comparison, Washington, DC February 3, 2003.

36  

Prengaman, R. D. Lead and Lead Refining: Committee on Scientific Assessment of Bullet Lead Elemental Composition Comparison, Washington, DC March 3, 2003.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

size and the wire diameter (bullet caliber). For example, a 70-lb billet should produce about 114 ft of wire intended for .22 caliber ammunition, but the same billet should produce about 27 ft of wire if .45 caliber bullets are the intended product. The extruder die may have a single exit port that produces a single wire strand from the billet, or it may have multiple extrusion ports that produce several wires from a single billet. Several feet of the wire formed at the beginning of the extrusion process may be discarded and recycled into a future billet.37

In brief, the wire is used as feed for a cutter, which consists of a machine that automatically introduces the wire into a cutting device to produce slugs, small cylinders of lead whose length and mass are close to those of the final bullet. The slugs are stored in large bins that may hold substantial quantities of slugs from different wires.

The binned slugs are fed into hoppers that feed the presses that form the bullets. Although it is not a true swaging process, this term is commonly encountered in the literature describing the process. Thus formed, the bullets are then tumbled, sometimes lubricated, and stored in bins.38,39 For some bullet types, a metal jacket is added.

Production of Loaded Ammunition

The loaded ammunition, which is sometimes referred to as rounds or cartridges, consists of a brass case that is charged with primer and powder and into which the bullet is pressed. Bullets and cases from bins are fed into hoppers, and the process of ammunition production proceeds in an automated fabrication machine. The product is sent directly to the packaging operation or is placed in large bins for later packaging.40,41

Packaging and Distribution

The bullet manufacturer packages the ammunition in boxes for shipment. The box typically is labeled with a stamp that refers to the “boxing lot,” which may be recorded as a date or simply a number. In some manufacturing plants,

37  

Frost, G. E. Ammunition Making, Chapter 3. National Rifle Association of America, Washington, DC 1990, 25–43.

38  

Frost, G. E. Ammunition Making, Chapter 3. National Rifle Association of America, Washington, DC 1990, 25–43.

39  

In some cases, bullets may be washed, rinsed, and plated in addition to being tumbled and lubricated. Each step can introduce further mixing of bullets from different lead wires and discrete sections of lead wire.

40  

Green, K. D. Introduction to the Bullet Manufacturing Process: Committee on Scientific Assessment of Bullet Lead Elemental Composition Comparison, Washington, DC February 3, 2003.

41  

Frost, G. E. Ammunition Making, Chapter 3. National Rifle Association of America, Washington, DC 1990, 25–43.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

the boxing lot number refers to the date the ammunition was loaded; in others, the date or number is not necessarily related to a particular stage in the production process. A typical box contains 20–50 cartridges, but some units or boxes are larger, depending on product line and caliber. For example, .22 long rifle “value packs” are commonly sold in 550-round boxes, and 100-round boxes of 9 × 19 mm ammunition have recently become common at larger retailers.42 The boxes are arranged in larger shipping units (such as cartons, crates, and pallets) and shipped to jobbers, distributors, wholesalers, or large retailers.

Attempts to obtain details on the shipping and distribution processes for loaded ammunition were unsuccessful and therefore are not clearly understood by the committee. For example, the committee has no evidence that distribution from a given manufacturer is regional as has been suggested in one report.43 Similarly, the frequency and size of shipments are unknown, but they are expected to vary widely, depending on the customer and the type of ammunition. However, it is reasonable to assume that high-turnover ammunition (for example, .22 caliber) is shipped more frequently than others and in larger quantities.

The committee has a similar lack of knowledge about retail dispersion of boxes. For example, it is not known whether first-in-first-out sales occur—that is, whether older shipments are arranged on shelves to be sold first.

COMPOSITIONAL INFORMATION

Multiple steps are required to move from bullet production to boxes of ammunition, and manufacturers vary in their processing of materials leading to bullet formation. In addition, storage times before actual packaging and shipping depend heavily on caliber; for example, high-production munitions, such as .22 caliber, probably move more rapidly from slug production to shipping than less-common munitions.

Homogeneity

There is much debate of the homogeneity of the lead “source.” It is unclear whether macro- and microscale inhomogeneities are present at some or all of the stages of lead and bullet production and if such inhomogeneities would affect CABL. The poor definition and understanding of the term “source” causes additional confusion. These topics are clarified below.

Melt. It is reasonable to assume that a given batch of molten lead exhibits sufficient mixing (such as convective stirring because of the heating process)

42  

Shotgun News Special Interest Publications, Peoria, IL May 20, 2003. A collection of firearms related advertisements for retailers and wholesalers.

43  

Randich, E.; Duerfeldt, W.; McLendon, W.; and Tobin, W. Foren. Sci. Int. 2002, 127, 174–191.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

for compositional homogeneity to develop quickly in the melt, assuming that there are no additions to the molten vat during pouring. Some constituents—such as Sb, As, and Sn—oxidize in air, and their loss or flotation to the surface is expected to take place slowly. However, the rate of compositional change is unlikely to be significant relative either to the rate of casting of billets or to the uncertainty of the concentrations of these materials. The assumption that the rate of compositional change is insignificant is supported by the small surface area exposed to air relative to the total mass of the melt.

Pigs, Ingots, and Billets. The homogeneity of ingots, pigs, and other large blocks of smelted lead is not an issue, because they are always remelted before billets are cast. Inhomogeneity of billets can arise from two factors. First, a billet may be cast in two stages, with the second stage long enough after the first for a measurable compositional difference to exist, depending on the constancy of the melt between the two pours that finalize billet production. Second, solutes inevitably segregate to the center of the billet during solidification.

Wires, Slugs, and Bullets. The extrusion process used to produce the wire from a billet is thought to negate the inhomogeneity due to segregation during solidification because the flow of the solid is turbulent as the billet enters the mouth of the die. Uniformity along the length of wire has not been substantiated. However, Koons and Grant have sampled wires produced from billets from a pour and found that concentrations remained constant (that is, within analytical precision) over several billets.44 Small compositional differences may exist along the length of the wire as a result of several factors. Segregation of material at the end of the billet mold may enrich the less refractory constituents in the lead, and detectable segregation will diminish as the impurity level decreases. If this segregation occurs, it still might not contribute to compositional differences along the length, because several feet of the first length of wire extruded are discarded and returned to a scrap bin. If multiple billets are loaded into an extruder, a continuous, single wire is extruded, but is cut into separate wires where the change of billets takes place.45 It is not clear from the data available whether the concentration of Sb is segregated in the billet or wire. While a paucity of data also exists for the spatial dependence of concentration of the other impurities along the length of wire (or in the billet), their significantly lower concentration should make spatial inhomogeneities less likely. It is reasonable to assume that cutting the wire to produce the slugs and pressing the slugs to form the final bullets produce no substantial segregation of elements in the lead.

Mixing of Slugs, Bullets, and Loaded Ammunition. Some manufacturers

44  

Koons, R. D. and Grant, D. M. J. Foren. Sci. 2002, 47, 950–958.

45  

Frost, G. E. Ammunition Making, Chapter 3. National Rifle Association of America, Washington, DC 1990, 25–43.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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use multiple cutting machines with distinct wire feeds to simultaneously produce slugs that are collected in a common slug bin. Similarly, a given production run may require sequential cutting of several wires and collection in a common bin. Thus, if wires are not of the same composition, a bin can contain slugs with a finite number of distinct compositions and if slugs from previous runs went unused at the start of the cutting of new wires, they contribute to the mixing of slugs of different compositions in a bin.

The slug bins are emptied into hoppers that feed the bullet-shaping presses, and the bullets formed may be collected in bullet bins before they are fitted into cases to form loaded ammunition. “Tail-in-tail-out” mixing can occur in the bins if their full contents are not used in a single production run of ammunition. The mixing with previously formed bullets will not occur if the pressed bullets are used immediately (without storage in bullet bins) in ammunition production.

The loaded ammunition can be routed directly to a packaging area, in which case no additional mixing occurs. However, loaded ammunition is sometimes stored temporarily in ammunition bins, where batch mixing and tail-in-tail-out procedures that contribute to mixing can occur.

The likelihood of mixing in the various bins described above is supported by the compositional analyses conducted on the bullets in a given box of ammunition.46 It is routinely found that a single box contains multiple distinct compositional groupings—as many as 14.47

Boxes, Crates, and Distribution. The boxes of ammunition are generally stamped with a box lot number. Depending on the manufacturer, this lot number may only reflect the packaging date, may be a direct indication of the date and shift during which the ammunition was loaded, or may be a code indicating packing date and shift, which can be traced through the manufacturer’s internal records to one or more shifts of loading operations. A stamped date does not reflect the date of pouring of billets, extrusion of wire, or formation of bullets. If filled boxes are stored on shelves because of overruns, boxes of different runs (with different dates) may be mixed in larger shipping units. Thus, a large-volume shipping unit for more commonly used ammunition might or might not contain only boxes with the same lot number and date.

As noted previously, distribution of boxes, crates, pallets, and other quantities of ammunition is poorly understood; there is minimal documentation to assist in establishing general trends. It is clear that distribution can lead to varied scenarios regarding retail dispersion of bullets from a distinct compositional group.

46  

Peters, C.; Havekost, D. G.; Koons, R. D. Crime Lab. Digest 1988, 15(2), 33–38.

47  

Peele, E. R.; Havekost, D. G.; Peters, C. A.; Riley, J. P.; Halberstam, R. C.; and Koons, R. D. In Proceedings of the International Symposium on the Forensic Aspects of Trace Evidence June 24–28, 1991, pp 57–68.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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THE “SOURCE”

When the metal compositions of two bullets are analytically indistinguishable, it is commonly suggested that they may have originated in the same “source.” It might be good to replace that vague term with “compositionally indistinguishable volume of lead” (CIVL). The CIVL, produced during one production run at one point in time, is at least as large as the sample taken for analysis. From the current understanding of the bullet production process, CIVL can refer to different tangible products associated with the manufacturing cycle. At its largest, the CIVL may be a vat of molten lead whose composition is not altered during the pouring of billets. Similarly, the CIVL may consist of a series of billets that were poured before the vat composition was altered by, for example, the addition of more molten lead to replenish the vat. At the very least, a CIVL may consist of several wires. The ramifications of identifying bullets whose compositions are analytically indistinguishable and their possible association with a single CIVL are discussed later in this chapter.

COMPOSITIONAL ANALYSIS OF BULLET LEAD AS EVIDENCE IN THE LEGAL SYSTEM

This section discusses the legal aspects of CABL evidence. Knowledge of the lead and bullet manufacturing processes underlies the proper interpretation of CABL evidence. The topics covered here include admissibility standards (including evaluation of match data) and pretrial discovery.

ADMISSIBILITY STANDARDS

The admissibility of CABL raises issues concerning expert testimony and relevance.

Expert Testimony

Experts are called by the prosecution to testify to the fact of matching and, in most cases, the evidentiary implication of a match. Federal Rule of Evidence 702 governs the admissibility of expert testimony in federal trials:

If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise, if (1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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In Daubert v. Merrell Dow Pharmaceuticals, Inc.,48 the Supreme Court interpreted an earlier version of Rule 702 to require that scientific evidence meet a reliability test. The Court wrote that “in order to qualify as ‘scientific knowledge,’ an inference or assertion must be derived by the scientific method. Proposed testimony must be supported by appropriate validation—i.e., ‘good grounds,’ based on what is known. In short, the requirement that an expert’s testimony pertain to ‘scientific knowledge’ establishes a standard of evidentiary reliability.”49 The Court held that the Frye test,50 which required that a novel scientific technique be generally accepted in the relevant scientific community as the sole condition for admissibility,51 had been superseded by Rule 702 of the Federal Rules of Evidence.

Under the Daubert analysis, the trial court must make “a preliminary assessment of whether the reasoning or methodology underlying the testimony is scientifically valid and of whether that reasoning or methodology properly can be applied to the facts in issue.”52 In performing this “gatekeeping function,” the trial court may consider a number of factors: whether the theory or technique can be and has been tested,53 whether it has been subjected to peer review and

48  

509 U.S. 579 (1993). See Margaret A. Berger, The Supreme Court’s Trilogy on the Admissibility of Expert Testimony, in Federal Judicial Center, Reference Manual on Scientific Evidence 9 (2d ed. 2000); David L. Faigman et al., Modern Scientific Evidence ch. 1 (2d ed. 2002); 1 Paul C. Giannelli & Edward J. Imwinkelried, Scientific Evidence ch. 1 (3d ed. 1999).

49  

509 U.S. at 590. The Court also commented that “under the Rules the trial judge must ensure that any and all scientific testimony or evidence admitted is not only relevant, but reliable.” Id. at 589. “In short, the requirement that an expert’s testimony pertain to ‘scientific knowledge’ establishes a standard of evidentiary reliability.” Id. at 590. In footnote 9, the Court elaborated: “We note that scientists typically distinguish between ‘validity’ (does the principle support what it purports to show?) and ‘reliability’ (does application of the principle produce consistent results?)…. [O]ur reference here is to evidentiary reliability—that is, trustworthiness…. In a case involving scientific evidence, evidentiary reliability will be based upon scientific validity.”

50  

Frye v. United States, 293 F. 1013, 1014 (D.C. Cir. 1923). See Paul C. Giannelli, The Admissibility of Novel Scientific Evidence: Frye v. United States, a Half-Century Later, 80 Columbia L. Rev. 1197 (1980).

51  

As noted below, “general acceptance” continues as a factor under Daubert but not the sole criterion for admissibility as under Frye.

52  

509 U.S. at 592-93. In a later passage, the Court wrote that “the Rules of Evidence—especially Rule 702—do assign to the trial judge the task of ensuring that an expert’s testimony both rests on a reliable foundation and is relevant to the task at hand. Pertinent evidence based on scientifically valid principles will satisfy those demands.” Id. at 597. See also Fed. R. Evid. 104(a) (“Preliminary questions concerning … the admissibility of evidence shall be determined by the court….”).

53  

Id. at 593 (“Ordinarily, a key question to be answered in determining whether a theory or technique is scientific knowledge that will assist the trier of fact will be whether it can be (and has been) tested. ‘Scientific methodology today is based on generating hypotheses and testing them to see if they can be falsified; indeed, this methodology is what distinguishes science from other fields of human inquiry.’ Green 645. See also C. Hempel, Philosophy of Natural Science 49 (1966) (‘[T]he statements constituting a scientific explanation must be capable of empirical test’); K. Popper, Conjectures and Refutations: The Growth of Scientific Knowledge 37 (5th ed. 1989) (‘[T]he criterion of the scientific status of a theory is its falsifiability, or refutability, or testability’) (emphasis deleted).”).

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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publication,54 a technique’s known or potential error rate, the existence and maintenance of standards controlling the technique’s operation,55 and a technique’s general acceptance in the relevant scientific community.56 Those factors, however, are neither dispositive nor exhaustive. The Court emphasized that the Rule 702 standard is “a flexible one.”

The Court followed with General Electric Co. v. Joiner57 and Kumho Tire Co. v. Carmichael58 to make up what is now known as the Daubert trilogy. Daubert and its progeny have come to be viewed as establishing a stringent standard of admissibility.59 In Weisgram v. Marley Co.,60 the Supreme Court remarked: “Since Daubert, … parties relying on expert evidence have had notice of the exacting standards of reliability such evidence must meet.”61 More-

54  

Id. 593-94 (“Another pertinent consideration is whether the theory or technique has been subjected to peer review and publication. Publication (which is but one element of peer review) is not a sine qua non of admissibility; it does not necessarily correlate with reliability, and in some instances well-grounded but innovative theories will not have been published. Some propositions, moreover, are too particular, too new, or of too limited interest to be published. But submission to the scrutiny of the scientific community is a component of ‘good science,’ in part because it increases the likelihood that substantive flaws in methodology will be detected. The fact of publication (or lack thereof) in a peer reviewed journal thus will be a relevant, though not dispositive, consideration in assessing the scientific validity of a particular technique or methodology on which an opinion is premised.”) (citations omitted).

55  

Id. at 594.

56  

Id. (“Widespread acceptance can be an important factor in ruling particular evidence admissible, and ‘a known technique which has been able to attract only minimal support within the community,’ …may properly be viewed with skepticism.”).

57  

522 U.S. 136 (1997) (specifying that the admissibility decision is to be reviewed on appeal under an abuse-of-discretion standard).

58  

526 U.S. 137 (1999). In Kumho, the Court extended Daubert’s reliability requirement to nonscientific expert testimony under Rule 702: “Daubert’s general holding—setting forth the trial judge’s general ‘gatekeeping’ obligation—applies not only to testimony based on ‘scientific’ knowledge, but also to testimony based on ‘technical’ and ‘other specialized’ knowledge.” Id. at 141.

59  

See Rider v. Sandoz Pharm. Corp., 295 F.3d 1194, 1202 (11th Cir. 2002) (“The district court, after finding that the plaintiffs’ evidence was unreliable, noted that certain types of other evidence may have been considered reliable, including peer-reviewed epidemiological literature, a predictable chemical mechanism, general acceptance in learned treatises, or a very large number of case reports.”); Jerome P. Kassirer and Joe S. Cecil, Inconsistency in Evidentiary Standards for Medical Testimony: Disorder in the Courts, 288 J. Am. Med. Assn. 1382, 1382 (2002) (“In some instances, judges have excluded medical testimony on cause-and-effect relationships unless it is based on published, peer-reviewed, epidemiologically sound studies, even though practitioners rely on other evidence of causality in making clinical decisions, when such studies are not available.”).

60  

528 U.S. 440 (2000) (reviewing a summary judgment in a wrongful death action against a manufacturer of an allegedly defective baseboard heater).

61  

Id. at 455. See also Brooke Group, Ltd. v. Brown & Williamson Tobacco Corp., 509 U.S. 209, 242 (1993) (“When an expert opinion is not supported by sufficient facts to validate it in the eyes of the law, or when indisputable record facts contradict or otherwise render the opinion unreasonable, it cannot support a jury’s verdict.”).

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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over, some federal courts have read the Daubert trilogy as inviting a “reexamination even of ‘generally accepted’ venerable, technical fields.”62

In 2000, Rule 702 was amended63 to codify Daubert and Kumho.64 The Advisory (drafting) Committee’s note to that rule supplements the Daubert factors with other considerations: whether the underlying research was conducted independently of litigation, whether the expert unjustifiably extrapolated from an accepted premise to an unfounded conclusion, whether the expert has adequately accounted for obvious alternative explanations, whether the expert was as careful as he or she would be in professional work outside of paid litigation, and whether the field of expertise claimed by the expert is known to reach reliable results.65

The Daubert decision is restricted to federal trials; it does not apply to other jurisdictions.66 Thus, states are free to determine their own standards for admissibility of expert testimony, even in the 40 or so jurisdictions that have adopted evidence rules based on the Federal Rules of Evidence. Many jurisdictions have adopted the Daubert framework.67 Moreover, other jurisdictions had rejected the Frye test before the Daubert decision,68 and many of these now look to Daubert for guidance.69

62  

United States v. Hines, 55 F. Supp. 2d 62, 67 (D. Mass. 1999). See also United States v. Hidalgo, 229 F. Supp. 2d 961, 966 (D. Ariz. 2002) (“Courts are now confronting challenges to testimony … whose admissibility had long been settled.”). Nevertheless, other courts seem to apply a less stringent approach to some long accepted forensic techniques. See United States v. Crisp, 324 F.3d 261 (4th Cir. 2003) (fingerprint and handwriting comparison; compare majority and dissenting opinions).

63  

The following clause was added to Rule 702: “if (1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case.”

64  

Some courts believe the amendment went beyond Daubert and Kumho. See Rudd v. General Motors Corp., 127 F. Supp. 2d 1330, 1336-37 (M.D. Ala. 2001) (“[T]he new Rule 702 appears to require a trial judge to make an evaluation that delves more into the facts than was recommended in Daubert, including as the rule does an inquiry into the sufficiency of the testimony’s basis (‘the testimony is based upon sufficient facts or data’) and an inquiry into the application of a methodology to the facts (‘the witness has applied the principles and methods reliably to the facts of the case’). Neither of these two latter questions that are now mandatory under the new rule … were expressly part of the former admissibility analysis under Daubert.”).

65  

Fed. R. Evid. 702 advisory committee’s note (2000).

66  

Daubert, 509 U.S. at 587 (“We interpret the legislatively enacted Federal Rules of Evidence as we would any statute.”).

67  

Alaska, Colorado, Connecticut, Idaho, Indiana, Kentucky, Massachusetts, Nebraska, New Hampshire, New Mexico, Oklahoma, South Dakota, Tennessee, and West Virginia. See 1 Paul C. Giannelli & Edward J. Imwinkelried, Scientific Evidence § 1-13 (3d ed. 1999).

68  

Arkansas, Delaware, Georgia, Iowa, Montana, North Carolina, Ohio, Oregon, Rhode Island, South Carolina, Texas, Utah, Vermont, and Wyoming. See id. at § 1-14.

69  

E.g., Nelson v. State, 628 A.2d 69, 73 (Del. 1993) (“Our decisions [in prior cases] are consistent with the Supreme Court’s decision in Daubert.”); State v. Foret, 628 So. 2d 1116, 1123 (La. 1993) (“Past decisions of this court have espoused similar sentiments [as Daubert]…”).

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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Nevertheless, some jurisdictions have retained the Frye rule.70 Because Federal Bureau of Investigation (FBI) examiners testify in state trials, the Frye general-acceptance standard may apply to CABL in some cases.71

Relevance and Its Counterweights

Relevance is the threshold issue for all evidence. Federal Rule 401 defines relevant evidence as “evidence having any tendency to make the existence of [a material or consequential fact] more probable or less probable than it would be without the evidence.” Rule 401’s standard does not require that the evidence make a consequential (material) fact “more probable than not” (“preponderance of evidence”) but only that the material fact (for example, the identity of a perpetrator) be more probable or less probable with the evidence than without the evidence.72

Rule 402 makes relevant evidence admissible in the absence of a rule of exclusion, and Rule 403 specifies circumstances under which a trial court is permitted to exclude relevant evidence. Rule 403 reads: “Although relevant, evidence may be excluded if its probative value is substantially outweighed by the danger of unfair prejudice, confusion of the issues, or misleading the jury, or by considerations of undue delay, waste of time, or needless presentation of cumulative evidence.” In Daubert, the Supreme Court noted that “expert evi-

70  

E.g., People v. Leahy, 882 P.2d 321, 323 (Cal. 1994) (The “Kelly formulation [of Frye under the Cal. Evid. Code] survived Daubert….”); People v. Miller, 670 N.E.2d 721, 731 (Ill. 1996) (“Illinois follows the Frye standard for the admission of novel scientific evidence.”); Burral v. State, 724 A.2d 65, 80 (Md. 1999) (Despite Daubert, “we have not abandoned Frye or Reed.”). Other Frye jurisdictions include Alabama, Arizona, Florida, Kansas, Michigan, Minnesota, Mississippi, Missouri, Nevada, New Jersey, New York, Pennsylvania, and Washington. See 1 Giannelli & Imwinkelried, Scientific Evidence § 1-15 (3d ed. 1999).

71  

Some jurisdictions adhere to a third approach, known as the relevance approach. See State v. Peters, 534 N.W.2d 867, 873 (Wis. Ct. App. 1995) (“Once the relevancy of the evidence is established and the witness is qualified as an expert, the reliability of the evidence is a weight and credibility issue for the fact finder and any reliability challenges must be made through cross-examination or by other means of impeachment.”); State v. Donner, 531 N.W.2d 369, 374 (Wis. Ct. App. 1995) (“[B]efore Daubert, the Frye test was not the law in Wisconsin. To that extent, Wisconsin law and Daubert coincide. Beyond that, Wisconsin law holds that ‘any relevant conclusions which are supported by a qualified witness should be received unless there are other reasons for exclusion.’ Stated otherwise, expert testimony is admissible in Wisconsin if relevant and will be excluded only if the testimony is superfluous or a waste of time…. Assuming that Daubert in its application represents something beyond Walstad, we observe that we … are bound to follow our supreme court case law.”) (citations omitted).

72  

In some situations, the relevance of evidence depends on science—or at least knowledge outside the common experience of laypersons. See Fed. R. Evid. 401 advisory committee’s note (federal drafters noted that relevance decision are based on “experience or science, applied logically to the situation at hand”).

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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dence can be both powerful and quite misleading because of the difficulty in evaluating it. Because of this risk, the judge in weighing possible prejudice against probative force under Rule 403 of the present rules exercises more control over experts than over lay witnesses.”73 As suggested by that passage, scientific evidence is often cited for its potential to mislead the jury because it may “assume a posture of mystic infallibility in the eyes of a jury of laymen.”74 Furthermore, expert testimony using such terms as “match” can be misleading unless explained.

CABL Evidence in the Courts

Although CABL evidence has been admitted in evidence for 30 years, there are relatively few published cases on the technique. The overwhelming majority of them are homicide prosecutions,75 some of which are capital cases. Because there are few federal homicide statutes, CABL evidence is most commonly used in state prosecutions. The courts that have addressed the admissibility of CABL evidence have admitted it—at least in the published cases.76 CABL evidence is often used in cases in which numerous other items of evidence are introduced, but courts have sometimes indicated that it played an important role in securing a conviction.77

The published cases reveal a wide variety of interpretive conclusions with respect to CABL evidence. In many cases, the experts apparently have not, in their testimony, recognized the limitations of such evidence. We first describe some of the testimony and then turn to a description of permissible conclusions.

73  

Daubert, 509 U.S. at 595 (quoting Weinstein, Rule 702 of the Federal Rules of Evidence is Sound; It Should Not Be Amended, 138 F.R.D. 631, 632 (1991)).

74  

United States v. Addison, 498 F.2d 741, 744 (D.C. Cir. 1974). See also People v. King, 72 Cal. Rptr. 478, 493 (Ct. App. 1968) (“Jurors must not be misled by an ‘aura of certainty which often envelops a new scientific process, obscuring its currently experimental nature.’”).

75  

But see United States v. Davis, 103 F.3d 660 (8th Cir. 1996) (federal trial for armed bank robbery and using a firearm during a crime of violence).

76  

As the committee was completing its report, a federal district court excluded CABL evidence under the Daubert standard. United States v. Mikos, 2003 WL 22922197, No. 02 CR 137 (N.D. Ill. Dec. 9, 2003).

77  

See Earhart v. Johnson, 132 F.3d 1062, 1068 (5th Cir. 1998) (federal habeas review) (“Given the significant role the bullet evidence played in the prosecution’s case, we shall therefore assume Earhart could have made a sufficient threshold showing that he was entitled to a defense expert under Texas law.”); State v. Noel, 697 A.2d 157, 160 (N.J. Super. App. Div. 1997) (“Before we address the expert-testimony problems, we note that without that testimony, the State’s proofs consisted entirely of the two eyewitness identifications and defendant’s possession of nine-millimeter Speers bullets. … Thus, with respect to the eyewitnesses, both of whom were found in the house where a suspect was believed to be and both of whom were evidently involved with drugs, one recanted and the testimony of the other was contradicted by an apparently disinterested witness.”), rev’d, 723 A.2d 602 (N.J. 1999).

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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In some cases, experts have testified only that two exhibits are “analytically indistinguishable,”78 but it is often unclear whether that was the only conclusion rendered at trial. In other cases, experts concluded that samples could have come from the same “source” or “batch”;79 in still others, experts stated that the samples came from the same source.80

The testimony in a number of cases goes further and refers to a “box” of ammunition (usually 50 loaded cartridges, sometimes 20). For example, two specimens

  • Could have come from the same box,81

  • Could have come from the same box or a box manufactured on the same day,82

  • Were consistent with their having come from the same box of ammunition,83

78  

See Wilkerson v. State, 776 A.2d 685, 689 (Md. 2001) (The expert “concluded that all six items contained similar lead material and were probably manufactured by Remington Peters. The lead material in one bullet and one projectile was analytically indistinguishable, as was the lead in one bullet and the other two projectiles.”).

79  

See State v. Krummacher, 523 P.2d 1009, 1012-13 (Or. 1974) (The “analyses showed that the bullet could have come from the same batch of metal as the group of bullets which was taken from defendant’s home but not from the same batch as any of the other groups.”).

80  

See United States v. Davis, 103 F.3d 660, 673-74 (8th Cir. 1996) (“He also concluded that these bullets must have been manufactured at the same Remington factory, must have come from the same batch of lead, must have been packaged on or about the same day, and could have come from the same box.”); People v. Lane, 628 N.E.2d 682, 689-90 (Ill. App. 1993) (“He testified that the two bullets were analytically indistinguishable. Special Agent Riley opined that the two bullets came from the same source and that the match was as good as he had ever seen in his twenty years with the FBI.”) (emphasis added).

81  

See State v. Strain, 885 P.2d 810, 817 (Utah App. 1994) (“Riley concluded that one of the bullets taken from the victim’s body and the bullet taken from the gun Strain possessed when he was arrested could have come from the same box of ammunition.”); State v. Jones, 425 N.E.2d 128, 131 (Ind. 1981) (“Agent Riley stated that the bullet from the victim could have come from the same box of ammunition as did the two cartridges that had bullets that matched.”).

82  

See State v. Grube, 883 P.2d 1069, 1078 (Idaho 1994) (“He further opined that the shot shells from which the crime scene pellets came could have come from the same box as the shot shells from Grube; or were from boxes manufactured at the same place on or about the same date.”); People v. Johnson, 499 N.E.2d 1355, 1366 (Ill. 1986) (“samples ‘would commonly be expected to be found among bullets within the same box of cartridges with compositions just like these, and that [that is, another box of cartridges close in composition] could best be found from the same type and manufacture [sic] packaged on the same day.’”); State v. Earhart, 823 S.W.2d 607, 614 (Crim. App. Tex. 1991) (“He later modified that statement to acknowledge that analytically indistinguishable bullets which do not come from the same box most likely would have been manufactured at the same place on or about the same day; that is, in the same batch.”).

83  

See State v. Reynolds, 297 S.E.2d 532, 534 (N.C. 1982) (“Further, neutron activation analysis revealed that the bullets taken from Morgan and Stone and the ammunition found with defendant were of the same chemical composition, consistent with their having come from the same box of ammunition.”).

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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  • Probably came from the same box,84

  • Must have come from the same box or from another box that would have been made by the same company on the same day.85

The transcript in State v. Earhart contains the following testimony: “We can—from my 21 years experience of doing bullet lead analysis and doing research on boxes of ammunition down though the years I can determine if bullets came from the same box of ammunition….”86 In People v. Kennedy, the examiner testified: “If you are comparing two and they have exactly the same composition that’s what you do, expect they came out of the same box.”87

Several other (and different) statements appear in the published cases. An early case reported that the specimens “had come from the same batch of ammu-

84  

See Bryan v. Oklahoma, 935 P.2d 338, 360 (Okla. Crim. App. 1997) (FBI agent Peele testified “that the bullets from the victim, the Lincoln, the rifle, and Bryan’s room all came from the same source, were manufactured in the same batch, and probably came in the same box.”).

85  

See United States v. Davis, 103 F.3d 660, 666-67 (8th Cir. 1996) (“An expert testified that such a finding is rare and that the bullets must have come from the same box or from another box that would have been made by the same company on the same day.”; the court wrote that “expert testimony demonstrated a high probability that the bullets spent at the first robbery and the last robbery originated from the same box of cartridges.”); Commonwealth v. Daye, 587 N.E.2d 194, 207 (Mass. 1992) (Agent Riley testified that “two bullet fragments found in Patricia Paglia’s body came from the same box of ammunition or from different boxes that were manufactured at the same place on or about the same date as a bullet retrieved from the basement of the Rye house. Riley further testified that three other bullets found in Patricia Paglia’s body ‘could have come from the same box of ammunition’ as the two bullet fragments mentioned above.”); State v. King, 546 S.E.2d 575, 584 (N.C. 2001) (Kathleen Lundy “opined that, based on her lead analysis, the bullets she examined either came from the same box of cartridges or came from different boxes of the same caliber, manufactured at the same time.”).

86  

Testimony of John Riley, State v. Earhart, No. 4064, Dist Ct. Lee County, 21st Judicial Dist., Texas, Transcript at 5248-49; State v. Earhart, 823 S.W.2d 607 (Crim. App. Tex. 1991). See also Transcript at 5258 (“Well, bullets that are—that have analytically indistinguishable compositions or compositions that are generally similar typically are found within the same box of ammunition and that is the case that we have here. Now, bullets that are the same composition can also be found in other boxes of ammunition, but it’s most likely those boxes would have been manufactured at the same place on or about the same date.”); Testimony of John Riley, State v. Mordenti, Florida: “It’s my opinion that all of those bullets came from the same box of ammunition. Now, I have to put one condition on that. And that is if they didn’t come from the same box of ammunition … then they came from another box that was manufactured at the same place on or about the same date. And the reason I have to say that is when these cartridges were manufactured at Remington Peters, they obviously loaded more boxes than one that had this composition of bullets in it.” Transcript at 480.

But see testimony of Charles Peters, Commonwealth v. Wilcox, Kentucky, Feb. 28, 2002 (Daubert hearing: “We have never testified, to my knowledge, that that bullet came from that box. We’d never say that. All we are testifying is that that bullet, or that victim fragment or something, the bullet, either came from that box or the many boxes that were produced at the same time.” Transcript at 1-2.)

87  

Testimony of Ernest Peele, People v. Kennedy, No. 95CR4541, Dist. Ct., El Paso County, Colorado, July 31, 1997, Transcript.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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nition: they had been made by the same manufacturer on the same day and at the same hour.”88 One case reports the expert’s conclusion with a statistic.89 In another case, the expert used the expressions “rare finding”90 and “a very rare finding”.91 In still another case, the expert “opined that the same company produced the bullets at the same time, using the same lead source. Based upon Department of Justice records, she opined that an overseas company called PMC produced the bullets around 1982.”92

In recent years, testimony appears to have become more limited. A 2002 FBI publication states the conclusion as follows: “Therefore, they likely originated from the same manufacturer’s source (melt) of lead.”93 Testimony to the same effect has also been proffered.94

Recent laboratory reports reviewed by the committee contain the following conclusion: “The specimens within a composition group are analytically indistinguishable. Therefore, they originated from the same manufacturer’s source (melt) of lead.”95 Another laboratory report used more cautious language: “This is consistent with the specimens within those groups originating from the same manufacturer’s source (melt) of bullet lead.”96

The most recent edition of the FBI Handbook of Forensic Sciences contains the following comment: “Differences in the concentrations of manufacturer-controlled elements and uncontrolled trace elements provide a means of differentiating among the lead of manufacturers, among the leads in individual manu-

88  

Brown v. State, 601 P.2d 221, 224 (Alaska 1979) (emphasis added) (unclear whether an FBI examiner was the expert).

89  

State v. Earhart, 823 S.W.2d 607, 614 (Crim. App. Tex. 1991) (“He concluded that the likelihood that two .22 caliber bullets came from the same batch, based on all the .22 bullets made in one year, is approximately .000025 percent, ‘give or take a zero.’ He subsequently acknowledged, however, that the numbers which he used to reach the .000025 percent statistic failed to take into account that there are different types of .22 caliber bullets made each year—.22, .22 long, and .22 long rifle. Agent Riley ultimately testified that there could be several hundred thousand bullets per batch, but with some variation in the elemental composition within the batch.”).

90  

United States v. Davis, 103 F.3d 660, 666 (8th Cir. 1996) (“The bullets from the box found in the Nissan were determined to be analytically indistinguishable from the bullets recovered at the 74th Street Mid City Bank and the 42nd Street Mid City Bank. An expert testified that such a finding is rare and that the bullets must have come from the same box or from another box that would have been made by the same company on the same day.”).

91  

Id. at 667.

92  

People v. Villarta, 2002 Cal. App. Unpub. Lexis 4776 (murder).

93  

Charles A. Peters, The Basis for Compositional Bullet Lead Comparisons, 4 Forensic Sci. Communications No. 3, at 5 (July 2002) (emphasis added).

94  

Testimony of Charles Peters, Commonwealth v. Wilcox, Kentucky, Feb. 28, 2002, Transcript (trial testimony): “Well, bullets that are analytically indistinguishable likely come from the same molten lead sources of lead, uh, as opposed to bullets that have different composition come from different, uh, melts of lead.”

95  

State v. Anderson, Mahoning County, Ohio, March 19, 2001, Dr. Diana Grant (examiner).

96  

People v. Garner, Colorado, Dec. 11, 1998, Kathleen M. Lundy (examiner).

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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facturer’s production lines, and among specific batches of lead in the same production line of a manufacturer.”97

The opinions in some cases indicate that prosecutors and courts have overstated the probative impact of matching evidence. For example, in its appellate division brief in State v. Noel,98 “the State asserted that this testimony is reliable scientific proof not only that the bullets ‘came from the same source of lead at the manufacturer’ but were ‘sold in the same box.’” Part of the problem in this case was the prosecutor’s summation, which made this argument. The intermediate appellate court believed that the argument was prejudicially misleading,99 but the New Jersey Supreme Court, although conceding that the argument may have been “excessive,” held that it might pass as “fair comment.”100 Similarly, in United States v. Davis,101 the court wrote that “the evidence made it more probable than not that the expended bullets originated from the cartridge box found in the Nissan.”102 The committee has made several recommendations (see infra) concerning how trial testimony should be presented.

97  

FBI Handbook of Forensic Sciences 36 (rev. 1999). An earlier edition stated: “Analysis may determine that the composition of the bullet and or fragment is identical to the composition of the recovered ammunition. Although circumstantial, lead composition information is often useful to link a suspect to a shooting, and similar information may be determined from an analysis of shot-pellets and slugs.” F.B.I. Handbook of Forensic Science 57 (rev. 1994).

98  

723 A.2d 602, 608 (N.J. 1999) (dissent).

99  

State v. Noel, 697 A.2d 157, 165 (N.J. Super. App. Div. 1997):

Beyond the inherent problems with the expert testimony itself, we are also persuaded that the prosecutor’s “snowflake or fingerprint” comment during closing must necessarily have further misled the jury in its task of assessing the probative value of Peters’ identical-composition testimony. We recognize that to some extent the comment did not actually mischaracterize the testimony that the batches were most likely unique, although there was no real evidential basis for the “millions of batches” comment. The point, of course, is that the relationship of batches to billets to bullets was already confusing enough and insufficiently developed by the expert testimony. Thus, the clear import of the fingerprint and snowflake comparison was to suggest to the jury a scientific certainty in the inference that defendant had possessed both sets of bullets and to suggest to the jury a conclusiveness of that inference that clearly was not warranted. We conclude, therefore, that no matter how indulgently we might view the problems with the expert testimony itself, the prosecutor’s summation, uncorrected by the court on defendant’s objection, injected a high degree of prejudice into this trial.

100  

State v. Noel, 723 A.2d 602, 607 (N.J. 1999):

In overruling defendant’s objection in the prosecutor’s final statement to the analogy between snowflakes and bullets, the trial court characterized the statement as a “metaphor.” In his own closing argument, defense counsel, apparently anticipating the prosecutor’s summation, argued that many boxes contain bullets matching the ones at issue. That argument directed the jury’s attention to the issue that concerns the dissent, “whether too many bullets were in circulation to justify any conclusive inference of guilt.” During the course of the trial, moreover, defense counsel vigorously cross-examined Peters. Finally, nothing prevented defense counsel from introducing evidence contradicting Peters’s testimony or from requesting a charge on the jury’s use of that testimony if it found the evidence to be unreliable or misleading.

101  

103 F.3d 660 (8th Cir. 1996).

102  

Id. at 674. The expert testified only that the bullets were analytically indistinguishable, that such a finding is rare, and “that the bullets must have come from the same box or from another box that would have been made by the same company on the same day.” There may have been hundreds or thousands of other boxes manufactured that day.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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EVALUATION

CABL involves three steps: chemical analysis, statistical analysis, and the interpretation of data derived from them. As one commentator noted when evidence based on neutron activation analysis (NAA) was first introduced, “most of the legal problems surrounding NAA [now inductively coupled plasma-optical emission spectroscopy (ICP-OES)] do not involve its validity as a technique of chemical analysis. Rather, interpretation of the results of the chemical analysis—the relevance of the results to a particular legal issue—causes most of the difficulties.”103 Because the analytical technique (ICP-OES) has not been an issue, we deal here with the third step—relevance and interpretation.104

Relevance

Evidence that crime scene bullets and loaded cartridges associated with a suspect came from the same melt is relevant under the definition of Rule 401, which is a low standard.105 It has a “tendency to make the existence of any fact that is of consequence to the determination of the action [that is, the identity of the perpetrator] more probable … than it would be without the evidence.”106

103  

Comment, The Evidentiary Uses of Neutron Activation Analysis, 59 Cal. L. Rev. 997, 998 (1971). Accordingly, the “qualifications of the expert as an analytical chemist do not necessarily establish his competence to interpret the legal relevance of his measurements.” Id. at 1031.

104  

As discussed in Chapter 2, the analytical method, if properly applied, is reliable. The reliability of ICP has not been an issue in the cases or in the literature. E.g., State v. Noel, 697 A.2d 157, 162 (N.J. Super. App. Div. 1997) (“To begin with, we have no doubt that ICP analysis of lead bullets is a process adequately accepted by the scientific community and producing sufficiently reliable results to warrant the admission of expert testimony regarding the test and the test results.”), rev’d on other grounds, 723 A.2d 602 (N.J. 1999).

105  

See State v. Noel, 697 A.2d 157, 162 (N.J. Super. App. Div. 1997) (“Establishment of the fact that the two sets of bullets came from the same source of lead clearly enhances the probative weight that a jury would be inclined to accord to mere similarity of calibre and manufacture.”), rev’d on other grounds, 723 A.2d 602 (N.J. 1999).

106  

Fed. R. Evid. 401 (emphasis added). See Margaret A. Berger, Procedural Paradigms for Applying the Daubert Test, 78 Minn. L. Rev. 1345, 1357 (1994) (“A match [sometimes] does have a ‘tendency to make the existence of any fact that is of consequence to the determination of the action more probable … than it would be without the evidence.’ We allow eyewitnesses to testify that the person fleeing the scene wore a yellow jacket and permit proof that a defendant owned a yellow jacket without establishing the background rate of yellow jackets in the community. Jurors understand, however, that others than the accused own yellow jackets. When experts testify about samples matching in every respect, the jurors may be oblivious to the probability concerns if no background rate is offered, or may be unduly prejudiced or confused if the probability of a match is confused with the probability of guilt, or if a background rate is offered that does not have an adequate scientific foundation.”) (footnotes omitted).

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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The critical issues, however, are how probative such a finding is107 and how that probative value is conveyed to the jury.

There are two aspects of relevance in this context: the likelihood that crime scene bullets came from the same CIVL as the defendant’s bullets and the likelihood that the crime scene bullets came from the defendant.108

Scientifically Supportable Conclusions (Same Melt)

A description of the probative force of evidence is given by the likelihood ratio for such evidence. The likelihood ratio for bullet lead match data is the probability that two bullets would match if they came from the same CIVL divided by the probability that they would match (coincidentally or through error) if they came from different CIVLs. If the likelihood ratio is much larger than 1, the fact of a match is strong evidence that the bullets came from the same CIVL; if not, the evidence is weak.109

To illustrate how this concept could be used quantitatively, assume for the sake of discussion that the probability that two bullets would match if they came from the same CIVL (the sensitivity of the test) is 0.90, and the probability of a match by coincidence or error of two bullets from different CIVLs (the false positive probability) is 1 in 500 or 0.002 The likelihood ratio110 would then be 0.90/0.002 = 450. That can be interpreted in two ways: the probability of such a match is 450 times greater if the bullets came from the same melt than if they came from different melts, and the odds that the bullets came from the same melt are 450 times greater with the match evidence than without it (that is, there is no

107  

“It is probable that the jury’s assessment of the strength of the link would be affected by whether defendant had a handful of similar bullets out of 1,000, or out of 10,000, or out of 100,000, or out of a million.” State v. Noel, 697 A.2d 157, 163 (N.J. Super. App. Div. 1997), rev’d on other grounds, 723 A.2d 602 (N.J. 1999).

108  

The second issue is discussed below as “defendant as provider of bullets.”

109  

See Richard O. Lempert, Modeling Relevance, 75 Mich. L. Rev. 1021, 1025-26 (1977) (“Where the likelihood ratio for an item of evidence differs from one, that evidence is logically relevant. This is the mathematical equivalent of the statement in Federal Rules of Evidence (FRE) 401 that ‘relevant evidence’ is ‘evidence having any tendency to make the existence of any fact that is of consequence to the determination of the action more probable or less probable than it would be without the evidence. Hence, evidence is logically relevant only when the probability of finding that evidence given the truth of some hypothesis at issue in the case differs from the probability of finding the same evidence given the falsity of the hypothesis at issue. In a criminal trial, if a particular item of evidence is as likely to be found if the defendant is guilty as it is if he is innocent, the evidence is logically irrelevant on the issue of the defendant’s guilt.”).

110  

Here, the likelihood ration is not defined strictly as statisticians would use the term, but in a way that has been acceptable in court.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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evidence either way on matching).111 With either interpretation, the evidence in this example would strongly support the conclusion that the bullets came from the same CIVL.

However, in reality the sensitivity and the false positive rate of CABL as applied by the FBI are not available. Therefore, the interpretation can be given only in qualitative terms: the probability of a match is greater if the bullets came from the same CIVL than if they came from different CIVLs, and the odds that the bullets came from the same CIVL are greater with the matching evidence than without it. Note that the witness may not testify as to the probability or odds that the bullets came from the same CIVL but only, in the first interpretation, as to the relative increase in probability of a match if the bullets came from the same vs different CIVLs or, in the second interpretation, as to the relative increase in the odds that the bullets came from the same CIVL if they matched vs no evidence of match status.

The admissibility of the above-described evidence depends on whether the assumption made above, namely, that bullets from the same CIVL have a greater probability of having the same composition as bullets from different CIVLs, has sufficient scientific support to be reliable. That requires us to look at two assumptions currently made in the use of CABL: homogeneity within CIVLs (which affects the likelihood that two bullets from the same CIVL have the same composition), and homogeneity between CIVLs (which affects the likelihood that two bullets from different CIVLs have the same composition.)

111  

The latter formulation is an application of Bayes’s theorem. In a convenient formulation, the theorem provides that:

Posterior odds given the evidence = prior odds × likelihood ratio.

In this context, the posterior odds given the evidence are the odds that the two bullets came from the same melt given that they are analytically indistinguishable (“match”); the prior odds are the odds that the bullets came from the same melt based on the other evidence in the case (such as evidence indicating that the bullets may have come from the defendant’s supply); and the likelihood ratio is, as already defined, the probability that the bullets would match if they came from the same melt divided by the probability that they would match if they came from different melts. When FBI examiners find two bullets that match, they have a basis for testifying that the likelihood ratio is greater than 1, but they cannot properly testify as to the posterior probabilities that the bullets came from the same melt. Because they have no knowledge of the rest of the case, they have no basis for picking prior probabilities, which would be necessary for opining on posterior probabilities. Moreover, even if they had knowledge of the context of the case, testimony based on their prior probabilities would not necessarily be relevant or appropriate, because the jurors might have different priors, and the choice of a prior is not a matter of expertise. The most an expert can validly say is that the odds that the bullets came from the same melt are increased by the evidence of elemental similarity; this is true regardless of the level of prior odds. See State v. Spann, 617 A.2d 247 (N.J. 1993) (improper for expert to testify to posterior probabilities using her own prior).

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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Homogeneity within CIVLs. FBI expert witnesses frequently imply or state in their testimony that if bullets came from the same melt,112 they will always match, that is, the test has perfect sensitivity. A single study by FBI personnel tested the assumption of homogeneity of melts and found it to be reasonable (sensitivity more than 90 percent).113 A study by critics of the assumption (Randich et al.) concludes that lead from a single melt can be inhomogeneous.114 Possible reasons for this conclusion were discussed. However, no measure of sensitivity is given in the study, and the authors did not publish the standard deviations of their measurements, so it cannot be determined to what extent the differences found were analytically indistinguishable. Despite the debate, the existence of inhomogeneity in a melt should not seriously affect the probative value of the evidence and may, in some respects, enhance it. We discuss the reason for this below.

Even if there is considerable inhomogeneity in a melt, two bullets that come from one melt and that have the same composition must have come from a subpart of the melt that was homogeneous. Fewer bullets can be made from a subpart than from the whole melt, so the fact of inhomogeneity within a melt, if it exists, does not weaken the inferences that can be legitimately made about matching bullets. However, because the degree of inhomogeneity will in general not be known, it must be assumed, conservatively, that the number of bullets of the same composition is such as would be produced from an entire melt. The principal risk of inhomogeneity is a false negative—two bullets declared not to match when they come from the same melt. Under our system of justice, such errors are less objectionable than false positives because they would usually favor a suspect.

The committee has addressed the issue of homogeneity by defining a source not as a melt, but rather as a CIVL (compositionally indistinguishable volume of lead), which may be limited to a subpart of a melt.

False Positives. False positives occur when a laboratory error or a coincidence (two CIVLs with analytically indistinguishable composition) causes two bullets to match. The rate of laboratory error is unknown because the FBI Laboratory does not have a program of testing by an external agency that has been designed to assess the proficiency of its examiners. The FBI’s internal testing program does not appear to be designed to determine an error rate. If we

112  

In this case the term “melt” is used rather than CIVL because that is the term used by the FBI in their testimony. “Melt” will also be used on other occasions in this chapter when the original source uses the term.

113  

Robert D. Koons and Diana M. Grant, Compositional Variation in Bullet Lead Manufacture, 47 J. Forensic Sci. 950 (2002) (of 456 comparisons of bullets from common sources, differences were statistically and analytically significant in only 33).

114  

Erik Randich et al., A Metallurgical Review of the Interpretation of Bullet Lead Compositional Analysis, 127 Forensic Sci. Int’l 174 (2002).

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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assume the laboratory’s error rate is in fact low (an assumption not currently grounded in evidence and made here only for the sake of the argument at hand), then the overwhelming contribution to the denominator of the likelihood ratio is CIVLs that are coincidentally identical in their composition.

The frequency of coincidentally identical CIVLs is unknown. Based on available data, the frequency of coincidental matches has been studied by the FBI. The data used in the FBI study have been further analyzed by the committee as described in Chapter 3. Those analyses have found some evidence supporting the assumption that the frequency of coincidental false positives is quite low. However, the FBI’s study is weakened because (1) the data used by the FBI were culled by the Bureau from a larger data set consisting of a collection of bullets analyzed by the FBI over a period of 14 years, and the method of culling may have introduced statistical bias; (2) the 2-SD overlap and range overlap method used by the FBI for declaring a match do not have quantifiable error rates (although approximate error rates can be calculated as in Chapter 3); and (3) the FBI study has been neither peer-reviewed nor published.115

Daubert/Kumho Factors

The Daubert/Kumho factors previously referred to provide an indication of whether proposed expert testimony is sufficiently reliable to be admissible at trial. They expressly apply to the federal courts, to the state courts in those states that have adopted Daubert, and are likely to be influential to some degree in those states retaining the Frye standard. We briefly examine below the assumptions of homogeneity and low false positive error rates from this perspective.116

Whether the theory can be and has been tested. Both homogeneity and a low false positive rate are assumptions that can be and have been tested, as described above and in Chapter 3. As noted in those discussions, the tests of both assumptions have weaknesses. For the reasons stated above, the assumption of homogeneity within a melt is not crucial to the value of the evidence. The

115  

The authors of the Randich study claim in conclusory fashion that the rate of false positives is high but do not calculate a rate. If their data and assertions are accepted, the rate for their Table 3 would be about 1 in 500. The difference between the FBI rate and the Randich rate may be due in part to the fact that the Randich data are from only two manufacturers whereas the FBI data are from all manufacturers and cover a much longer period.

116  

One federal court of appeals has admitted CABL evidence under the Daubert test. United States v. Davis, 103 F.3d 660 (8th Cir. 1996). However, the court did not have the information that the committee had available to it. Moreover, the court overstated the probative value of the evidence. The court wrote: “The evidence made it more probable than not that the expended bullets originated from the cartridge box found in the Nissan.” Id. at 674. As the committee was completing its report, a federal district court excluded CABL evidence under the Daubert standard. United States v. Mikos, 2003 WL 22922197, No. 02 CR 137 (N.D. Ill. Dec. 9, 2003).

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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assumption of a low false positive rate is important. As the analysis in Chapter 3 indicates, the statistical method used by the FBI may be leading to a false positive rate much higher than that assumed by examiners. A statistical method can be chosen to minimize the false positive rate, but this is always done at the expense of a higher false negative rate. Additional testing would be needed to fully satisfy the Daubert/Kumho testing requirement.

Whether the theory has been subjected to peer review and publication. There are very few peer-reviewed articles on homogeneity and the rate of false positive matches in bullet lead composition.117 Early articles focused on NAA118 and other techniques,119 used fewer elements in the analysis, and did not address the question of statistical interpretation. Moreover, some of the published articles appeared in FBI publications.120 Outside reviews have only recently been published.121 Because this evidence is less than conclusive and the case volume that utilizes this technique is low, the subject has not received the broad review that DNA testing and some other techniques have. Again, more such work would be needed to provide a strong basis for this admissibility factor.

Whether the theory has a known error rate. The false positive probability due to coincidence has been estimated by the FBI, as noted above, but has not been published. Furthermore, as discussed in Chapter 3, this estimate is not

117  

Like many forensic techniques, CABL evidence gained admissibility before the demanding standards of Daubert were operative. The FBI has attempted to satisfy these standards through its recent publications and by referring the issue to this committee.

118  

E.g., Vincent P. Guinn, NAA of Bullet-Lead Evidence Specimens in Criminal Cases, 72 J. Radioanal. Chem. 645 (1982); Vincent Guinn & M.A. Purcell, A Very Rapid Instrumental Neutron Activation Analysis Method for the Forensic Comparison of Bullet-Lead Specimens, 39 J. Radioanal. Chem. 85 (1977); A. Brandon & G. F. Piancone, Characterization of Firearms and Bullets by Instrumental Neutron Activation Analysis, 35 Int’l J. App. Radiat. Isot. 359 (1984).

119  

See M.A. Haney & J.F. Gallagher, Differentiation of Bullets by Spark Source Mass Spectrometry, 20 J. Forensic Sci. 484 (1975); R.L. Brunelle, C.M. Hoffman & K.B. Snow, Comparison of Elemental Compositions of Pistol Bullets by Atomic Absorption: Preliminary Study, 53 J. A.O.A.C. 470 (1970).

120  

See C.A. Peters, D.G. Havekost, & R.D. Koons, Multi-Element Analysis of Bullet Lead by Inductively Coupled Plasma-Atomic Emission Spectrometry, 15 Crime Laboratory Digest 33 (1988); E.R. Peele et al., Comparison of Bullets Using the Elemental Compositions of the Lead Component, Proc. Int’l Sym. On the Forensic Aspects of Trace Evidence, Quantico, Va., 1991; Charles A. Peters, The Basis for Compositional Bullet Lead Comparisons, 4 Forensic Sci. Communications No. 3 (July 2002).

121  

See Raymond O. Keto, Analysis and Comparisons of Bullet Leads by Inductively-Coupled Plasma Mass Spectrometry, 44 J. Forensic Sci. 1020, 1026 (1999) (“This data suggests [sic] that when two element signatures match, it is unlikely that the bullets originated from different sources. The extent of each particular source (i.e., the number of identical boxes by each manufacturer) and the bullets available in a particular geographic area at a particular time are all unknown factors.”); Erik Randich et al., A Metallurgical Review of the Interpretation of Bullet Lead Compositional Analysis, 127 Forensic Sci. Int’l 174 (2002); William A. Tobin & Wayne Duerfeldt, How Probative Is Comparative Bullet Lead Analysis, 17 Crim. Justice 26 (Fall. 2002).

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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based upon an appropriately random sample of the bullet population. Laboratory error is another important factor in the false positive probability; the FBI has not estimated this factor and assumes it is essentially zero. In sum, the Daubert/ Kumho factor requiring a theory to have a known error rate is only partially satisfied.

The existence and maintenance of standards controlling the technique’s operation. The FBI has standards controlling the training of examiners, the laboratory protocol, and the statistical method for declaring a match. However, the laboratory protocol needs to be revised to reflect current practice.122 Moreover, the FBI does not have detailed standards governing the content of laboratory reports and the testimony that may be given by examiners. As a result, this Daubert/Kumho factor in significant part is not satisfied.

General acceptance in the relevant scientific or technical community. The analytical technique used (that is, previously NAA and now ICP-OES) has general acceptance of the scientific community for this sample type. However, to the committee’s knowledge the FBI is the only laboratory performing this type of lead analysis for forensic use, so any inquiry into “general acceptance” will not provide the broad consensus that this factor assumes. The fact that courts have generally admitted this testimony is not the equivalent of scientific acceptance, owing to the paucity of published data, the lack of independent research, and the fact that defense lawyers have generally not challenged the technique.123

The fact that the specifically mentioned Daubert factors are not fully satisfied does not mean that CABL evidence should not be admitted under the reliability standards of Rule 702. In Kumho Tire, the Court concluded “that a trial court may consider one or more of the more specific factors that Daubert mentioned when doing so will help determine that testimony’s reliability. But as the Court stated in Daubert, the test of reliability is “flexible,” and Daubert’s list of specific factors neither necessarily nor exclusively applies to all experts or in every case. Rather the law grants a district court the same broad latitude when it decides how to determine reliability as it enjoys in respect to its ultimate reliability determination.”124 However, the reliability and acceptance of the evidence would be strengthened if the FBI took the steps that the committee recommends.

122  

Conversations with FBI examiners indicate that crime bullets are compared one-to-one with the suspect’s bullets and not with compositional groups of the suspect’s bullets as specified by the present protocol.

123  

Attorneys have probably not challenged the evidence because the identifying link it provides to the same source is far from conclusive evidence that the defendant supplied the crime bullet. They often focus on the large number of bullets from a single melt rather than the technical intricacies of the matching process.

124  

Kumho Tire, 526 U.S. at 141-142 (emphasis in original).

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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Defendant as Provider of Bullets

As noted earlier, relevance in this context depends not only on an association between the crime scene bullet and the same melt as the suspect’s bullet but also on the further inference that this association suggests that the crime scene bullet came from the defendant. A conclusion that two bullets came from the same melt does not justify an expert in further testifying that this fact increases the odds that the crime bullet came from the defendant. The large number of bullets made from a single melt and the absence of information on the geographic distribution of such bullets125 precludes such testimony as a matter of expertise.126 Such an inference is a matter for the jury. An expert with distributional information might be able to provide such testimony to aid the jury.

The available data do not permit any definitive statement concerning the date of manufacture or the identity of the manufacturer based on elemental composition alone. However, in some cases, boxes with lot numbers are recovered, which may provide some information on this issue.127 In other cases, physical (as opposed to chemical) characteristics of crime bullets are observed, which

125  

See Jones v. State, 425 N.E.2d 128, 135 (Ind. 1981) (dissent) (“all retailers in a particular geographic area might consequently market bullets of similar composition”); State v. Noel, 697 A.2d 157, 163 (N.J. Super. App. Div. 1997) (“[T]he enhancement value to be placed on the same-batch conclusion must be basically a statistical probability exercise, that is, an assessment by the trier of fact of how much more likely it is that both sets of bullets were defendant’s because they not only matched in calibre and manufacture but also in composition. That assessment must necessarily depend on how many nine-millimeter bullets could have been produced from a single batch, what the likelihood is that those same bullets wound up for sale in the same geographical area, and what percentage of nine-millimeter bullets marketed in the Newark area came from Speers. Obviously, the strength of the link created by identical composition is a factor of how many bullets of identical composition were simultaneously available for sale in the Newark area, and, just as obviously, the statistical probability of defendant having possessed both sets of bullets declines as the number of identical bullets increases.”), rev’d on other grounds, 723 A.2d 602 (N.J. 1999).

126  

The absence of distributional information also makes it inappropriate for an expert to testify that the probability that two bullets came from the same source if the defendant did not fire the crime bullet was described by the number of bullets made from the source divided by the total number of bullets of that type made in some period, such as 1 year.

127  

State v. Freeman, 531 N.W.2d 190, 195 & n. 5 (Minn. 1995) (“This box of 50 cartridges contained the same loading code, 2TB90L, as the empty cartridge box found in the snowbank at the scene of Freeman’s arrest. This loading code indicated that the cartridges contained in both boxes were manufactured on February 9, 1982, during the second shift at Winchester’s plant located in East Alton, Illinois.”; “Also, both boxes were labelled with a Target price tag indicating a cost of $1.39.”). Lot numbers indicate the date of packaging, not the date the bullet was produced or the date the loaded cartridge was assembled.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

may augment the probative value of the evidence.128 Also, “matches” of multiple crime scene bullets to multiple suspect’s bullets from different CIVLs may add to the probative value of the evidence in a particular case.129 Similarly, a case with a “closed set” of suspects presents a different situation.130

PRETRIAL DISCOVERY

The need for pretrial disclosure of the nature and content of expert testimony is critical if the adversary system of trial is going to work. The American Bar Association (ABA) Standards note that the “need for full and fair disclosure is especially apparent with respect to scientific proof and the testimony of experts. This sort of evidence is practically impossible for the adversary to test or rebut at trial without an advance opportunity to examine it closely.”131 Never-

128  

Physical characteristics include, for example, the caliber of the bullet, the number of lands and grooves as well as their direction of twist, and whether the bullet was jacketed or not. In some cases, empty cartridge cases are found at crime scenes, which would reveal the caliber and manufacturer as well as other information. E.g., State v. Ware, 338 N.W.2d 707, 712 (Iowa 1983) (“wadcutter bullet removed from Tappa’s body”); State v. King, 546 S.E.2d 575, 583-84 (N.C. 2001) (Firearms examiner, who also testified in case, “determined that a spent round submitted to him, as well as the live rounds recovered during the investigation, were .22-caliber long-rifle bullets. According to Agent Wilkes, the live rounds he examined were similar in physical characteristics to the lead bullet projectile removed from the victim’s wrist.”); State v. Noel, 697 A.2d 157, 160 (N.J. Super. App. Div. 1997) (“A bag containing eighteen bullets was found in [defendant’s] locker. Nine of the bullets were nine-millimeter bullets stamped with the manufacturer’s name, Speers. The police had also recovered spent bullets and bullet casings at the crime scene. The shell casings were also stamped with the same manufacturer’s name.”), rev’d on other grounds, 723 A.2d 602 (N.J. 1999); State v. Krummacher, 523 P.2d 1009, 1012 (Or. 1974) (“The bullet found in Dorothy’s body was identified as being a .38 caliber lubaloy copper-washed Smith and Wesson type bullet manufactured by the Western Company, which went out of business three years prior to the crimes in question.”). The combination of physical characteristics and analytic indistinguishability can be powerful evidence in a particular case.

129  

Charles A. Peters, The Basis for Compositional Bullet Lead Comparisons, 4 Forensic Sci. Communications No. 3, at 5 (July 2002) (“Another factor that must be considered is a case where multiple shots of various calibers, manufacturers, and compositions are fired at a crime scene. If multiple compositions present in the crime-scene lead are analytically indistinguishable from lead groups in partial boxes of ammunition, it is much more likely that the crime-scene bullets came from those boxes than it is when only one compositional group is present.”).

130  

A “closed set” case is one in which the universe of suspects is limited—for example, only one of two persons could have fired the crime bullet, so differentiation between ammunition from them is the principal concern.

131  

Commentary, ABA Standards Relating to Discovery and Procedure Before Trial 66 (Approved Draft 1970). See also Paul C. Giannelli, Criminal Discovery, Scientific Evidence, and DNA, 44 Vanderbilt L. Rev. 791 (1991).

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

theless, pretrial discovery is often less extensive in criminal litigation than in civil cases.132

Federal Criminal Rule 16 governs discovery in federal trials. Four distinct provisions are relevant to expert testimony: scientific reports, summaries of experts’ expected testimony, other documents,133 and independent testing.134

Reports. Rule 16(a)(1)(F) makes the “results or reports of physical or mental examinations, and of scientific tests or experiments” discoverable. Under this provision, reports are discoverable if they are either material to the preparation of the defense or are intended for use by the prosecution as evidence in its case-in-chief at trial.135 Unfortunately, the rule does not specify the content of a laboratory report. While the measurement data (means and standard deviations) on CABL evidence are discoverable, it is more logical and of greater use to include these data in the laboratory report.

132  

Opponents of liberal discovery argue that criminal discovery will encourage perjury, lead to the intimidation of witnesses, and, because of the Fifth Amendment, be a one-way street. 2 C. Wright, Federal Practice and Procedure § 252, at 36-37 (2d ed. 1982). In the case of scientific evidence, however, these arguments against criminal discovery lose whatever force they might otherwise have. The first argument fails because “it is virtually impossible for evidence or information of this kind to be distorted or misused because of its advance disclosure.” Commentary, ABA Standards Relating to Discovery, supra, at 67. Moreover, it is extremely unlikely that an FBI expert will be subject to intimidation. See also 2 Wayne LaFave & Jerold H. Israel, Criminal Procedure § 19.3, at 490 (1984) (“Once the report is prepared, the scientific expert’s position is not readily influenced, and therefore disclosure presents little danger of prompting perjury or intimidation”). Finally, the self-incrimination clause presents little impediment to reciprocal prosecution discovery of scientific proof. See Williams v. Florida, 399 U.S. 78 (1970). In any event, it seems unlikely that defense experts will be retesting this type of evidence.

133  

Rule 16(1)(a)(E) (formerly 16(1)(a)(C)) makes documents in the government’s possession discoverable—such as bench notes and graphs that may not be part of the final report. See United States v. Armstrong, 517 U.S. 456, 463 (1996) (“Rule 16(a)(1)(C) authorizes defendants to examine Government documents material to the preparation of their defense against the Government’s case-in-chief”); United States v. Zanfordianno, 833 F. Supp. 429, 432 (S.D.N.Y. 1993) (“A narrow view of Rule 16(a)(1)(C) is inappropriate; failure to provide reasonably available material that might be helpful to the defense and which does not pose any risks to witnesses or to ongoing investigation is contrary to requirements of due process and to the purposes of the Confrontation Clause. If an expert is testifying based in part on undisclosed sources of information, cross-examination vouchsafed by that Clause would be unduly restricted.”).

134  

Independent testing has apparently not been a major issue in this context.

135  

Virtually all jurisdictions provide for the disclosure of scientific reports in the possession of the prosecution. Scientific reports also are discoverable under the ABA Standards and the Uniform Rules. ABA Standards for Criminal Justice 11-2.1(a)(iv) (3d ed. 1996) (“Any reports or statements made by experts in connection with the case, including results of physical or mental examinations and of scientific tests, experiments, or comparisons”); Unif. R. Crim. P. 421(a) (Approved Draft 1974) (“expert reports”). See also National Advisory Commission on Criminal Justice Standards and Goals, Courts, Standard 4.9(3) (1973).

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

The conclusions in laboratory reports should be expanded to include the limitations of CABL evidence.136 In particular, a further explanatory comment should accompany the laboratory conclusions to portray the limitations of the evidence. Moreover, a section of the laboratory report translating the technical conclusions into language that a jury could understand would greatly facilitate the proper use of this evidence in the criminal justice system.137 Finally, measurement data (means and standard deviations) for all of the crime scene bullets and those deemed to match should be included.

Summaries. Rule 16(a)(1)(G) requires the government, on defense request, to disclose a written summary of the testimony of the experts that it intends to use during its case-in-chief. The summary must describe the witnesses’ opinions, the bases of and reasons for the opinions, and the witnesses’ qualifications. This provision was intended to “expand federal criminal discovery” in order to “minimize surprise that often results from unexpected expert testimony, reduce the need for continuances, and to provide the opponent with a fair opportunity to test the merit of the expert’s testimony through focused cross-examination.”138 Although the ABA Standards recommend this type of discovery,139 most states do not have comparable provisions.

Conclusions. Like the NRC’s Committee on DNA Technology in Forensic Science, the present committee concludes that broad discovery is needed to the extent feasible: “The prosecutor has a strong responsibility to reveal fully to defense counsel and experts retained by the defendant all material that might be necessary in evaluating the evidence.”140 As one court put it,

136  

Professor Anna Harrison, Mount Holyoke College, during a symposium on discovery, remarked: “Then the information you are receiving is not scientific information. For a report from a crime laboratory to be deemed competent, I think most scientists would require it to contain a minimum of three elements: (a) a description of the analytical techniques used in the test requested by the government or other party, (b) the quantitative or qualitative results with any appropriate qualifications concerning the degree of certainty surrounding them, and (c) an explanation of any necessary presumptions or inferences that were needed to reach the conclusions.” Symposium on Science and the Rules of Legal Procedure, 101 F.R.D. 599, 632 (1984) (emphasis added).

137  

This recommendation will reduce the potentially misleading character of the evidence. See discussion of prosecution summary in State v. Noel, supra.

138  

Fed. R. Crim. P. 16, advisory committee’s note, reprinted at 147 F.R.D. at 473.

139  

ABA Standards for Criminal Justice 11-2.1(a)(iv) (3d ed. 1996) (“With respect to each expert whom the prosecution intends to call as a witness at trial, the prosecutor should also furnish to the defense a curriculum vitae and a written description of the substance of the proposed testimony of the expert, the expert’s opinion, and the underlying basis of that opinion.”).

140  

National Research Council, DNA Technology in Forensic Science 146 (1992). See also id. at 105 (“Case records—such as notes, worksheets, autoradiographs, and population databanks—and other data or records that support examiners’ conclusions are prepared, retained by the laboratory, and made available for inspection on court order after review of the reasonableness of a request.”). The 1996 DNA report contains the following statement on discovery: “Certainly, there are no strictly scientific justifications for withholding information in the discovery process, and in Chapter

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

“there are no scientific grounds for withholding information in the discovery process.”141

A statement of the limitations of CABL evidence should be included in the laboratory report. Providing an express statement of the limitations of the technique in the laboratory report not only provides notice to the parties, it affords substantial protection for experts from overreaching by attorneys. Experts are sometimes pressured by the prosecutor to “push the envelope”—not a surprising occurrence in the adversary system.142 ABA Criminal Justice Standard 3-3.3(a) states: “A prosecutor who engages an expert for an opinion should respect the independence of the expert and should not seek to dictate the formation of the expert’s opinion on the subject. To the extent necessary, the prosecutor should explain to the expert his or her role in the trial as an impartial expert called to aid the fact finders….” The commentary to this standard states: “Statements made by physicians, psychiatrists, and other experts about their experiences as witnesses in criminal cases indicate the need for circumspection on the part of prosecutors who engage experts. Nothing should be done by the prosecutor to cast suspicion on the process of justice by suggesting that the expert color an opinion to favor the interests of the prosecutor.”143

FINDINGS AND RECOMMENDATIONS

Finding: Variations among and within lead bullet manufacturers makes any modeling of the general manufacturing process unreliable and potentially misleading in CABL comparisons.

Recommendation: Expert witnesses should define the range of “compositionally indistinguishable volumes of lead” (CIVL) that could make up the source of analytically indistinguishable bullets, because of variability in the bullet manufacturing process.

   

3 we discussed the importance of full, written documentation of all aspects of DNA laboratory operations. Such documentation would facilitate technical review of laboratory work, both within the laboratory and by outside experts…. Our recommendation that all aspects of DNA testing be fully documented is most valuable when this documentation is discoverable in advance of trial.” National Research Council, The Evaluation of Forensic DNA Evidence 167-69 (1996).

141  

State v. Tankersley, 956 P.2d 486, 495 (Ariz. 1998).

142  

See Troedel v. Wainwright, 667 F. Supp. 1456, 1459 (S.D. Fla. 1986) (gunshot residue case) (“Next, as Mr. Riley candidly admitted in his deposition, he was ‘pushed’ further in his analysis at Troedel’s trial than at Hawkins’ trial. Furthermore, at the March 26th evidentiary hearing held before this Court, one of the prosecutors testified that, at Troedel’s trial, after Mr. Riley had rendered his opinion which was contained in his written report, the prosecutor pushed to ‘see if more could have been gotten out of this witness.’ When questioned why, in the Hawkins trial, he did not use Mr. Riley’s opinion that Troedel had fired the weapon, the prosecutor responded he did not know why.”), aff’d, 828 F.2d 670 (11th Cir. 1987).

143  

Commentary, ABA Criminal Justice Standard 3-3.3(a) at 59.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

Finding: The committee’s review of the literature and discussions with manufacturers indicates that the size of a CIVL ranges from 70 lbs in a billet to 200,000 lbs in a melt. That is equivalent to 12,000 to 35 million 40-grain, .22 caliber longrifle bullets from a CIVL compared with a total of 9 billion bullets produced each year.

Finding: CABL is sufficiently reliable to support testimony that bullets from the same compositionally indistinguishable volume of lead (CIVL) are more likely to be analytically indistinguishable than bullets from different CIVLs. An examiner may also testify that having CABL evidence that two bullets are analytically indistinguishable increases the probability that two bullets came from the same CIVL, versus no evidence of match status.

Recommendation: Interpretation and testimony of examiners should be limited as described above and assessed regularly.

Finding: Although it has been demonstrated that there are a large number of different compositionally indistinguishable volumes of lead (CIVLs), there is evidence that bullets from different CIVLs can sometimes coincidentally be analytically indistinguishable.

Recommendation: The possible existence of coincidentally indistinguishable CIVLs should be acknowledged in the laboratory report and by the expert witness on direct examination.

Finding: The available data do not support any statement that a crime bullet came from, or is likely to have come from, a particular box of ammunition, and references to “boxes” of ammunition in any form is seriously misleading under Federal Rule of Evidence 403.144 Testimony that the crime bullet came from the defendant’s box or from a box manufactured at the same time is also objectionable because it may be understood as implying a substantial probability that the bullet came from defendant’s box.

Finding: Compositional analysis of bullet lead data alone do not permit any definitive statement concerning the date of bullet manufacture.

144  

Testimony of Vincent Guinn, United States v. Jenkins, CR. No. 3:96-358, U.S. Dist. Ct., South Carolina, Columbia Div., Sept. 30, 1997, Transcript at 151 (Question: “Can you conclude if they match that the two bullets came from the same box of lead? [Answer:] “No, you can never do that. Every time they make a run from one particular melt, we are talking about a ton or more of lead involved. You can make an awful lot of bullets out of a ton of lead. So they get put in all these boxes and so on…. So, well, typically, for example, a one ton melt of lead will produce enough bullets, if it were just used itself, make enough bullets to fill something like 2,000 boxes of 50.”).

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
×

Finding: Detailed patterns of distribution of ammunition are unknown, and as a result an expert should not testify as to the probability that a crime scene bullet came from the defendant.145 Geographic distribution data on bullets and ammunition are needed before such testimony can be given.

Recommendation: The conclusions in laboratory reports should be expanded to include the limitations of compositional analysis of bullet lead evidence.146 In particular, a further explanatory comment should accompany the laboratory conclusions to portray the limitations of the evidence. Moreover, a section of the laboratory report translating the technical conclusions into language that a jury could understand would greatly facilitate the proper use of this evidence in the criminal justice system.147 Finally, measurement data (means and standard deviations) for all of the crime scene bullets and those deemed to match should be included.

145  

See State v. Noel, 697 A.2d 157, 162 (N.J. Super. App. Div. 1997) (“Nor was any testimony offered as to marketing, that is, whether, as seems likely, bullets from the same billets would be shipped together by the manufacturer and hence that there would be a concentration of such bullets in a specific geographical region.”), rev’d on other grounds, State v. Noel, 723 A.2d 602 (N.J. 1999).

The defense attorney in United States v. Jenkins, CR. No. 3:96-358, U.S. Dist. Ct., South Carolina, Columbia Div., Sept. 30, 1997, argued: “No company has still today provided us with any information from which we know whether all of this ammunition ended up in Columbia, South Carolina, or whether it was randomly distributed all over the country.” Transcript at 157.

Testimony of Charles Peters, Commonwealth v. Wilcox, Kentucky, Feb. 28, 2002, Transcript, (Daubert hearing & trial testimony). Question: “And do we have any information as to the geographic distribution of these bullets?” Peters: … “Uh, I, I don’t know the information. I, uh, obviously, uh, uh, to answer that question would bring somebody in from PMC.”

146  

Professor Anna Harrison, Mount Holyoke College, during a symposium on discovery, remarked: “Then the information you are receiving is not scientific information. For a report from a crime laboratory to be deemed competent, I think most scientists would require it to contain a minimum of three elements: (a) a description of the analytical techniques used in the test requested by the government or other party, (b) the quantitative or qualitative results with any appropriate qualifications concerning the degree of certainty surrounding them, and (c) an explanation of any necessary presumptions or inferences that were needed to reach the conclusions.” Symposium on Science and The Rules of Legal Procedure, 101 F.R.D. 599, 632 (1984) (emphasis added).

147  

This recommendation will reduce the potentially misleading character of the evidence. See discussion of prosecution summary in State v. Noel, supra.

Suggested Citation:"4. Interpretation." National Research Council. 2004. Forensic Analysis: Weighing Bullet Lead Evidence. Washington, DC: The National Academies Press. doi: 10.17226/10924.
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Since the 1960s, testimony by representatives of the Federal Bureau of Investigation in thousands of criminal cases has relied on evidence from Compositional Analysis of Bullet Lead (CABL), a forensic technique that compares the elemental composition of bullets found at a crime scene to the elemental composition of bullets found in a suspect’s possession. Different from ballistics techniques that compare striations on the barrel of a gun to those on a recovered bullet, CABL is used when no gun is recovered or when bullets are too small or mangled to observe striations. Forensic Analysis: Weighing Bullet Lead Evidence assesses the scientific validity of CABL, finding that the FBI should use a different statistical analysis for the technique and that, given variations in bullet manufacturing processes, expert witnesses should make clear the very limited conclusions that CABL results can support. The report also recommends that the FBI take additional measures to ensure the validity of CABL results, which include improving documentation, publishing details, and improving on training and oversight.

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