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Ballistic Imaging 2 Firearms and Ammunition: Physics, Manufacturing, and Sources of Variability A firearm is a dynamic system for delivering maximum destructive energy to a target, in the form of a high-velocity bullet, with minimum delivery of energy to the shooter. To that end, the firing of a firearm and the subsequent generation of ballistic toolmarks are the end results of processes that are simultaneously characterized by high uniformity and great variability. Modern firearms and ammunition manufacture relies heavily on the uniformity and interchangeability of component parts, yet each step in the production cycle presents an opportunity for microscopically fine differences from part to part. Likewise, the firing of a gun depends on the rapid and repeated performance of numerous mechanical steps that is designed to produce combustion, done in a controlled manner yet still not creating exactly identical conditions in repeated firings. In this chapter, we summarize the basic parts of firearms and ammunition (Section 2–A) and describe the physical processes that take place when a trigger is pulled and a gun is fired (2–B). These sections are not intended to be comprehensive examinations of the history and features of firearms and ammunition nor a complete catalogue of firearms products in current use. Rather, they provide context for the principal focus of this chapter: describing the types of toolmarks left on ballistics evidence by firing (2–C), particularly those that are typically imaged and input into ballistic image databases.1 We close in Section 2–D with brief descriptions of concepts in the manufacture of both firearms and ammunition. A general understanding of manufacturing is essential not only for an appreciation 1 More detailed information and images are available at http://www.firearmsid.com.
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Ballistic Imaging of the sources of variability in ballistic toolmarks, but also in assessing the feasibility of implementing technologies like wide-scale ballistic imaging or microstamping. 2–A ANATOMY OF FIREARMS AND AMMUNITION 2–A.1 Firearms Firearms come in a wide array of designs and specific makes, and each represents a complex assemblage of numerous constituent parts. In this section we focus on the parts most central to the basic firing assembly since the interest is in toolmark creation. Due to their widespread use in crime, we also discuss some terminology in the specific context of handguns, as in differentiating between revolvers and pistols. Barrels Gun barrels are manufactured from solid pieces of steel whose composition is carefully selected for its chemical and metallurgical properties. A first step of the process, drilling, results in a comparatively rough hole of uniform diameter extending from one end of the barrel to the other. Next the barrel is bored with a reamer, designed to produce as smooth a surface as possible on the inside of the barrel. The interior surface or bore bears numerous scars and scratches from this drilling process; it is these random imperfections—more so than subsequent steps—that are said to account for individual characteristics on fired bullets (Heard, 1997:124–125). Barrels are further subjected to a rifling process, creating a pattern of grooves on the inside the barrel. This rifling is essential to the firing accuracy of the weapon; as it is forced out of the barrel by gas pressure, the bullet impacts with the barrel rifling and is given a rotation—somewhat akin to the spin on a thrown football—that gives the bullet a more direct flight. Some weapons, typically shotguns, have no rifling (“smoothbore”). Most handguns and rifles have a spiral pattern of rifling to improve their accuracy. The rifling may be created by forcing a carbide button through the reamed barrel; it is the normal wear on this button, as many riflings are performed, that is said to impart individual microscopic variability in markings in the barrel (along with residual scars or imperfections from the original drilling). Additional steps in the process to finish a barrel include heat treating (to impart hardness) and cleaning. Across manufacturers, barrels can vary in two fundamental features, each of which are basic class characteristics (see Section 3–B.1). The first is the direction in which the grooves in the barrel twist, whether left- or right-handed. Most U.S. makers use a right twist, although Colt revolvers
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Ballistic Imaging are known for their left twist (Rinker, 2004:128). The second is the number of grooves that are cut into the barrel—normally at a depth of 0.004–0.006 inch—to create the rifling, and, correspondingly, the number of raised lands between those grooves. Historically, “no standard was established and makers used, normally, six, seven, or eight grooves”; this remains the usual range, although firearms have been fielded with as few as 2 and as many as 24 grooves (Rinker, 2004:130, 131). Barrels also vary in the degree of twist in the rifling, which affects how much rotation is put on bullets as they pass through the barrel and exit. Rinker (2004:127) observes that “few people agree on what is the proper twist. Some people want an over stabilized bullet from a fast twist. They claim best accuracy at all ranges. Other shooters believe a fast twist builds pressure and heat and they want a slow twist for minimum stability, and they have claims to back their theory.” Some firearms differ from conventional rifling with square-edged grooves, using polygonal rifling instead. “Polygonal rifling has no sharp edges,” and instead the raised lands in the barrel have a smooth, “rounded profile which can be difficult to discern when looking down the barrel. This type of rifling is almost exclusively manufactured using the hammer or swage process” (Heard, 1997:123). Chamber, Breech Face, and Firing Pin The rear section (away from the muzzle) of the barrel bore is known as the chamber; it is designed and sized to fit a specific caliber of cartridge (see Section 2–A.2). The part of the firearm against which a cartridge sits when it is placed in the chamber is the breech, and the whole assembly may be referred to as the breechblock or breech bolt. The specific surface of the breech that makes contact with the base of the cartridge is the breech face; Figure 2-1 depicts the breech faces of two firearms. The exact steps used to form the breech assembly can vary by manufacturer, and the breech face may vary in terms of the amount of filing or polishing done on it and whether any paint or other materials is applied to it. Basic filing can create gross striation marks in linear arrangements; in others, a rotary milling operation may be applied to the breech face surface, creating a pattern of concentric circles (American Institute of Applied Science, 1982:77). These steps are crucial to the theory of firearms identification as it is random imperfections created in these machining and filing processes that is said to make the surface (and the negative impressions of said surface, left on fired cartridge casings) unique. A hole drilled through the breech assembly holds the firing pin, a very hard steel rod that can be forced to protrude from the breech to strike the primer of a cartridge seated in the chamber. While most firing pins have a
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Ballistic Imaging FIGURE 2-1 Breech faces with firing pin holes: Two firearms. NOTES: The top image is the breech face of a Smith & Wesson firearm; the bottom image is the breech face of a Glock firearm. The shape of the firing pin hole for the Glock firearm indicates its characteristic rectangular firing pin. SOURCE: Excerpted from Tulleners (2001:Fig. 3-3).
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Ballistic Imaging small rounded end or nose, some have more distinctive shapes; in particular, Glock firearms are known for a rectangular firing pin. Firing pins are generally made on a standard screw machine. Like the breech face, the tip of the firing pin is subject to machining and filing steps that impart microscopic imperfections. Revolvers and Pistols Handguns may be divided into two basic types—revolvers and pistols—by the manner in which ammunition is loaded and cycled through the firearm. In a revolver, “the supply of ammunition is held in a cylinder at the rear of the barrel with each round having its own chamber,” and a ratchet mechanism is then used to cycle the cylinder to the next position (Heard, 1997:18). Revolvers may be further subdivided by the manner in which this cycling is performed. In single-action revolvers, the shooter manually cocks the hammer, pulling it back and setting the ratchet action in motion. A trigger pull then causes the hammer to drop and commence the firing process. More complex—and more common—double-action revolvers save a step: “A long continuous pull on the trigger cocks the hammer, rotates the cylinder, then drops the hammer all in one operation” (Heard, 1997:18). By comparison, pistols are self-loading, making use of ammunition “contained in a removable spring-loaded magazine housed within the grip frame.” Pistols have a single chamber, and individual rounds of ammunition are cycled into the chamber by mechanical means; pulling back the slide rearward until the breech face is behind the top round in the magazine, and then releasing it, forces the round forward and into the chamber for firing. After firing, the spent cartridge case is ejected “through a port in the side, or occasionally top, of the slide. At the end of its rearward motion, the spring-loaded slide moves forward[,] stripping a fresh round off the top of the magazine and feeding it into the rear of the barrel” (Heard, 1997:19). Pistols are often referred to as semiautomatic pistols (or semiautomatics); they are semiautomatic in that they are self-loading but require separate, distinct trigger pulls to fire different rounds. “Automatic” is used to describe “a weapon in which the action will continue to operate until the force is removed from the trigger or the magazine is empty.” Though a few fully automatic pistols have been marketed, they are rare “due to the near impossibility of controlling such a weapon [for accurate shots]…. Each shot causes the barrel to rise during recoil and before the firer has time to reacquire the target within the sights, the next round has fired”; consequently, “even at close range it is unusual for more than two shots to hit a man-sized target” (Heard, 1997:17, 18). For the objective of the recovery of ballistics evidence and imaging
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Ballistic Imaging thereof, the distinction between revolvers and pistols is vital: while pistols forcibly eject spent rounds, revolvers do not. Hence, casings may only be recovered at a crime scene involving a revolver if they are specifically emptied by a shooter (e.g., for reloading). Extractor and Ejector Both revolvers and pistols make use of an extractor, typically a small arm that fits over the rim of the cartridge. As the name implies, the extractor serves to pull a spent cartridge from the chamber so that a new cartridge can take its place. In a revolver, the extractor—which can remove all cartridges simultaneously by depressing the ejection rod (or extractor rod)—also has ratchet notches that advance the cylinder to the next chamber. In a semiautomatic pistol, however, the extractor removes the cartridge so that it makes contact with the ejector, typically a fixed protuberance that strikes the rim of the cartridge. Because these steps are performed very quickly, and with some speed and force, both the extractor and ejector mechanisms can leave marks on expended cartridge casings. 2–A.2 Ammunition Modern ammunition takes the form of integrated, self-contained cartridges, integrating three key elements in one unit: a bullet, the actual projectile that is expelled from the firearm’s barrel; propellant, which generates the force and pressure needed to put the bullet in motion and into flight; and a primer, which in modern usage is a volatile and pressure-sensitive chemical mixture that is responsible for igniting the propellant. Historically, with firearms of the 18th century, shooters had to assemble these components manually in order to reload, inserting black gunpowder, wadding, and a spherical lead ball into the gun’s barrel. With the intent of making reloading faster, early cartridges featured premeasured and prepackaged charges of powder, in small bags, but they still required an external source to provide a thermal “flash” to ignite the powder and fire the projectile. The innovation of the breechloader, by which the ammunition is loaded at the rear of the gun’s barrel, made modern integrated ammunition possible. Modern ammunition links these three components together, placing them inside an outer case. Ammunition is commonly identified based on the diameter of its bullet, for proper fitting with firearms barrels. The original designation of ammu-
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Ballistic Imaging nition size was by caliber: The unit of measurement was hundredths of an inch (e.g., .38 caliber corresponding to a bullet with diameter 0.38 inches). However, such caliber labels are only approximations, for example, a .38 caliber is actually 0.357 inches in diameter and a .40 caliber is actually 0.429 inches in diameter. Ammunition (and corresponding gun barrels) are also now identified using the metric system, such as 9mm or 10mm.2 Ammunition cartridges are primarily divided into two categories—rimfire and centerfire—depending on where the primer is located (and, correspondingly, where the gun’s firing pin strikes the cartridge during firing). We explain the distinction in the next section. Primer The use of a chemical primer to ignite the propellant dates back to the development of the percussion cap in the early 1800s, when it was discovered that striking a cap containing fulminate of mercury created a flame that could then move into the main charge of powder. Today, the exact chemical composition of primer mixtures can vary and remains proprietary. “Lead styphnate is the main ingredient,” generally, although individual primers may also include some of the following: “[trinitrotoluene (TNT)], lead or copper sulphocyanide, lead peroxide, sulfur, tetryl, barium peroxide, and barium nitrate” (Rinker, 2004:19). Ground glass may also be added as a “sensitizer,” to create friction when impacted by the firing pin (Matty, 1987:10). A primer mixture is a high explosive; working with it and placing the primer in the case are extremely sensitive parts of the ammunition manufacture process. Rimfire cartridges were first developed in the 1800s, and rimfire ammunition remains in heavy usage in .22 caliber cartridges. As the name implies, “the primer composition is spun into the rim of the cartridge case,” putting it in immediate contact with the powder propellant (Rinker, 2004:19–20). By comparison, centerfire ammunition has a cylindrical cap seated in the cartridge head that contains the primer mixture. The cap consists of a cup- 2 Care is needed with the use of the word “caliber.” Here, “caliber” is shorthand for the nominal caliber of the ammunition, which refers specifically to the diameter of the bullet. However, specific caliber of ammunition “refers to a name given to a cartridge representing the entire design of the cartridge as intended by the manufacturer, [including not only] the diameter of the bullet but the entire shape and size of the cartridge” (Moran, 2000:235). That is, a nominal-caliber ammunition group may include a wide variety of specific varieties that can vary significantly in their length, case design, powder charge, and so forth. Both “nominal caliber” and “specific caliber” are used to describe and label firearms as well, referring to the “group of firearms which share the same bore diameter” and the “name given to a firearm representing the specifically designed cartridge which will fit into the firearm,” respectively (Moran, 2000:235).
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Ballistic Imaging and-anvil combination and a pellet of primer mixture. During firing, the firing pin “compresses the primer composition between the cup and anvil,” causing a flame that passes through a hole or vent to ignite the propellant charge (Rinker, 2004:19). Practically, the development of the centerfire system “was the great milestone in weapon and ammunition development;” with it, “only the primer cup needed to be soft enough to be crushed by the firing pin,” freeing the main body of the cartridge case to be harder, providing “a gas seal for much higher pressures than could be obtained with rimfire ammunition” (Heard, 1997:11). Centerfire cartridges also developed, in part, due to the desire to reuse “the most expensive part of the cartridge, the case”; the centerfire configuration permits new primer assemblies to be inserted into expended casings (Matty, 1987:8). Given its purpose, the primer assembly must meet specific criteria. The primer mixture “must always have a uniform flash that is hot enough without being too violent. In other words, it must always consistently produce the proper amount of heat” (Rinker, 2004:20). Likewise, the material holding the primer—either the cartridge brass of the rim in a rimfire cartridge or the cup in a centerfire primer—must withstand the impact of the firing pin, the detonation of the primer, and the expansion of gas from the ignited propellant without rupturing. Centerfire primer cups are typically brass or nickel. Propellant Though it derives from centuries of development, a critical part of ammunition is subject to popular misunderstandings and mislabelings. It is commonly referred to as powder, tracing from ancient formulations of black powder and more modern incarnations of smokeless gunpowder. As Hatcher (1935:96) observes, powder “originally meant, and still does mean, fine dust; but at the present time we find substances called powder which do not in any manner resemble dust and which are not even finely divided.” Propellant is a more generic and more apt term for the substance used in modern ammunition. The individual particles of propellant may still be referred to as grains, even though they may not have a gritty or granular texture; however, the common use of grains to describe the exact quantity or charge of propellant in a cartridge has nothing to do with texture (a grain is a measured weight equal to 0.0648 grams). Fundamentally, a propellant is not devised to explode violently: It is designed to burn, and burn rapidly. As Rinker (2004:21) summarizes, “all gunpowder produces the force to move a projectile as the result of 3 things. (1) When it burns, it produces a huge quantity of gas. (2) As it burns, it produces a huge amount of heat. (3) After ignition, it creates its own oxygen and needs no outside air. All three are required. At first, the need
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Ballistic Imaging for heat may not be as obvious as the other two, but hot gas expands and requires more space then cold gas,” heightening the buildup of pressure in the gun’s chamber. Modern propellants are a form of nitrocellulose, first discovered in 1846 when cotton, nitric acid, and sulfuric acid were mixed. One pound of nitrocellulose-based powder contains 1.2–1.5 million foot pounds of stored chemical energy, in comparison with about 600,000 foot pounds of stored energy in one pound of the traditional saltpeter, charcoal, and sulfur combination of black powder (Rinker, 2004:23). “If ignited in an unconfined space,” nitrocellulose propellant will burn gently; if, however, combustion occurs in a confined space—as in a cartridge—“the heat and pressure built up will accelerate the rate of combustion exponentially” (Heard, 1997:76). The charge of propellant utilized in cartridges is carefully tuned to the caliber, bullet weight, barrel length, and desired performance of the ammunition. Chemical “moderating” agents or other additives (e.g., graphite or barium nitrate) are often used to control the burn rate of the propellant, and the mixes used in final propellants are “very tightly-controlled trade secrets” (Heard, 1997:59). Cartridge Cases Cartridge cases have traditionally been manufactured from brass, an alloy of copper and zinc, although other materials have been used; in particular, steel casings (coated with copper or a lacquer) were developed during World War II due to brass shortages, and steel cases remain in use in some countries because of their lower cost. Cartridge brass is almost universally of the same composition: a 70-to-30 or 75-to-25 alloy (in percentage of weight) of copper and zinc, respectively. This combination was developed, along with methods for working with it, as a result of the physical demands put on the case during the firing of a gun. As described below, a cartridge case expands during firing, pressing against the chamber walls to create a seal and containing the high-pressure gases created in firing. To accomplish this in situ deformation, the hardness of the cartridge brass must be precise so that the case retains its original shape and can be readily extracted from the breech. Too hard a starting brass and the case may crack during firing; too soft and it will expand and deform too much and be difficult to extract. Although there are a number of manufacturing processes currently used to produce cartridges, the salient features of the general manufacturing process are similar. Within the same case, thickness must also vary in particular ways, tailored to suit various tasks: maximum hardness in the rim (of a centerfire cartridge) in which the primer cap is seated, medium hardness with good elasticity in the central walls of the case, and softest at the neck or mouth end where the bullet is seated.
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Ballistic Imaging One modern manufacturing process for producing a centerfire case starts with brass rod or wire, in coils. A machine called a cold header, similar to the one used to make common nails, feeds in the rod or wire, cuts off a piece large enough to make one case, and transfers it to a cavity in the machine, where it is struck by a punch. This process forms the irregularly shaped cylindrical piece into a precise sort of button shape. The button is annealed (heated and then cooled) to reduce its hardness, and is then fed into a two-stage transfer press that transforms the cartridge blank into a low, wide cup. The half-formed cup is next pushed through a die or series of dies that draw the blank to its final shape and dimensions. Additional annealing, cleaning, and forming steps are done sequentially until the blank is in the final shape of the cartridge case. Bullets The last major component of the cartridge is the bullet or projectile. Bullets in modern ammunition can consist of a variety of metals. There are bullets made entirely of aluminum, steel, and sometimes brass; nonmetallic substances like rubber and wood have also been used to make bullets. However, to provide the needed weight for improved accuracy and performance, bullets most often contain some amount of lead. Bullets are designed for two basic purposes—penetration on impact with a target and perforation and expansion to increase damage—and the exact composition and construction of bullets are tailored to those purposes. An all-lead bullet is very soft and therefore expands rapidly on striking a target. Indeed, “pure lead is not used for lead bullets” precisely because “it is too soft [and] damages too easily in handling and loading”; antimony is most commonly added to lead as a hardening agent, though tin has also been used (Frost, 1990:27). Better penetration power at greater distances and accuracy can be attained by covering a lead core with a full jacket or partial jacket composed of a copper alloy. High-velocity, fully jacketed bullets are designed to penetrate deeply, while lower velocity jacketed bullets may tumble within the target and cause additional damage due to expansion. Mushrooming or expanding bullets, such as hollowpoints, are designed to transfer a maximum amount of energy to the target and to penetrate but not exit. The composition and design of bullets—along with what materials they do or do not strike—are important to forensic ballistics analysis as they affect what condition a recovered bullet will be in and hence how difficult it is to match to other evidence. A lubricant is applied to bullets before they are seated in cartridge casings; it acts to cut down on metal fouling of the bore, the deposition of particles or residues from the bullet (Frost, 1991:31). In centerfire cartridges, where “grease grooves” are created in the case by knurling, the
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Ballistic Imaging lubricant is usually a wax or heavy grease type; due to its placement, it must be a substance that will neither contaminate the powder nor react with lead or copper plating. 2–B THE FIRING OF A WEAPON: INTERNAL BALLISTICS The general concept of “ballistics” can be divided into separate stages; see Box 1-1. External ballistics (the flight path and behavior of the bullet between its exit from the barrel and its arrival at its target) and terminal ballistics (behavior of the bullet on striking a target) are both critical to complete firearms investigations. Our primary focus is on internal ballistics—the actions that occur between the pulling of the trigger and the bullet’s exit from the barrel of a firearm. Internal ballistics is “a series of actions or operations that every firearm must go through, whether .22 caliber revolver or a .50 caliber machine gun,” all of which occur in a time span on the order of 0.003 seconds (Rinker, 2004:1, 2). The trigger pull starts the mechanical process of allowing the firing pin to strike the primer of the chambered cartridge. The pressure from the firing pin creates a dent in the primer surface of the cartridge; more significantly, it causes a small explosion, the heat from which passes through the hole in the primer cap and into the main body of the cartridge. There, the charge of powder burns rapidly in a confined space, converting from a solid to a gas and exerting great pressure against all surfaces. “When the pressure has built up to a sufficient level, known as short shot, the bullet will start to move because the pressure is greater than the holding force of the case neck.” As the powder burn continues, “the pressure increases and the neck and body walls of the case expand to meet and grasp the inside chamber walls,” creating a seal and increasing the pressure acting on the bullet’s base, propelling it forward (Rinker, 2004:1). The bullet, being slightly larger than the barrel diameter, is forced to seat into the rifling (the lands and grooves) on the bore of the barrel, picking up rotation as it passes down the length of the barrel. While this sequence of events drives the bullet through the barrel and out of the firearm, forces are also at work on the head of the cartridge. Hatcher (1935:270, 272) describes the processes for a centerfire cartridge: When a primer is struck by the firing pin, the very brusque and powerful mixture that it contains explodes with violence, [causing the flame that ignites the powder charge]. But the explosion of the primer mixture also reacts in a backward direction onto the primer cup itself, and blows it part way out of the primer pocket, unless the primer is strongly crimped in place, as is done with some kinds of rifle ammunition. Then when the main charge ignites, the powder pressure inside the case forces the case
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Ballistic Imaging FIGURE 2-2 Breech face markings and firing pin impressions for three ammunition types and two firearm brands. NOTE: S&W=Smith & Wesson. SOURCE: Adapted from Tulleners (2001:Fig. 3-4). cartridge presses when it is being fired. These marks are quite pronounced on metal surfaces that have been finished by a file as is commonly done on the breech face of the average [semi]automatic pistol or revolver. Examined under a microscope this surface appears to consist of a number of ridges or scratches, and when the cartridge is fired, the primer, being of copper or brass, which is much softer than the steel of the breech face, will take the impression of these fine ridges. In gross appearance, features in the breech face impression may fall into some general categories depending on the specific filing or polishing steps used by the manufacturer. Straight filing creates linear features; other breech face impressions may feature cross-hatching or circular patterns. For example, Kennington (1995) documents the class of 9mm pistols for which the rotary cutting tool used in milling the breech face not only leaves distinctive arched markings that are impressed on the primer surface, but
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Ballistic Imaging may also be evident elsewhere on the cartridge head. Kennington suggests that the rifling characteristics from bullet evidence at a crime scene can be combined with evidence of arched markings on cartridge casings to rapidly identify the pistol make in question.3 Because breech face impressions are created by the pressure of firing, Tulleners (2001:3-2) notes that their detail “is dependent on cartridge chamber pressure and the type of breech face manufacture/condition. [Chamber pressure varies within caliber and depends on such factors as the bullet size and weight and the powder charge contained in the cartridge.] Lower pressure cartridges are not expected to consistently produce decent breech face impressions.” He adds that cartridge chamber pressure, bullet weight, and primer hardness “can vary to such an extent that an examiner will not be able to identify test 1 to test 2 when different ammunition is used in the same gun;” hence, “one of the cardinal rules in firearm examination is to test fire the gun with similar ammunition as the evidence ammunition if at all possible” (Tulleners, 2001:3-3). Firing Pin Impressions The firing pin impression on the surface of the primer provides important information on the general class of the firearm that discharged a casing. The shape of the “pit” marking the firing pin’s strike indicates the shape of the firing pin in the firearm (e.g., round, elliptical, rectangular). The firing pin impression will also bear the marks created by filing or smoothing the tip of the firing pin. “The point of the firing pin will have small ridges, and no two … firing pin points will be exactly alike,” conjectured Hatcher (1935:266). However, Burrard (1962:113) notes that “great caution is necessary” in distinguishing individual markings from grosser features of firing pin marks, which “often take the form of a number of small concentric rings.” Yet individual imperfections on the tip of the firing pin can be telltale: “Another by no means rare feature of a [firing pin] is the presence of a small ‘pimple’ on the extreme end,” and so the presence of a corresponding mark on one cartridge and the absence on another “would be proof positive that the [second] cartridge could not have been fired” from the same weapon as the first. For some guns and some firings, the firing pin impression may not be a clearly defined indentation on an otherwise flat surface. Instead, primer “flowback” may occur: a larger crater is created as the primer material 3 However, he cautions that “the arch-producing machine process … may not be the final breechface treatment at the factory. The breechface can still be broached, filed, sandblasted, tumbled and/or plated,” and residue buildup as a result of firing can obscure the arch markings.
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Ballistic Imaging around the pit is forced outward by gas pressure, partially flowing into the aperture in the breech from which the firing pin emerges. Though “flowback” is commonly attributed to firearms in which excessive pressure can build during firing, Kreiser (1995) suggests other explanations that also correspond to characteristics of the particular make of firearm. Among these is the diameter of the firing pin aperture: the wider the aperture, the more primer surface is unsupported (not positioned directly against another object) during firing and hence more likely to crater outward. In some firings, the firing pin may scrape against the surface of the primer as it is withdrawn. In these cases, the firing pin impression is not purely a mirror of the shape of the firing pin (e.g., circular) but has a drag mark trailing away from the main impression. Because drag marks may be repeated—that is, they may be a function of the behavior of the firing pin in a particular gun—they become important landmarks for traditional firearms identification and ballistic imaging alike, providing a benchmark to orient casings consistently. It is also important to note that the mechanics of firing is such that there is variability in the exact position where the firing pin impacts the cartridge across different firings; the pin may wobble slightly and strike at slightly different points and angles.4 In rimfire weapons, the firing pin strikes the brass of the outer rim of the cartridge head. As Hatcher (1935:68) observed, “[rimfire ammunition] takes a good impression showing the shape of the firing pin, but it does not often take a clear impression of the fine file marks and other irregular scratches on the breech block, which form the ‘finger-prints’ of the gun.” Accordingly, he noted that “when an empty rim fire cartridge is found at the scene of a shooting, it is often easy to say what type of arm was used; but it is seldom possible to identify a rimfire cartridge to a definite individual gun by the impression of the file marks it left on the head, as is so often done in the case of a center-fire cartridge.” Ejector Marks The ejector arms in automatic or semiautomatic firearms can vary in shape (e.g., rectangular, round, or triangular) and size; the footprint of the ejector determines the size and shape of the mark left by the ejector on the rim of the spent casing. Ejector marks can vary from tiny divots to 4 Fadal (1995) provides an unusual but vivid example of the difference that placement and angle of the firing pin strike can have on the resulting marks. The Hi-Standard Model DM-101 is a .22 caliber derringer handgun that is double-barreled; however, the same rectangular firing pin is used to initiate the firing in each of the two barrels. The difference in the way the same pin hits the (rimfire) casings in the two barrels—one using the top part of the pin and the other the lower—is sufficiently large that an examiner cannot match firings from one barrel to firings from the second barrel on the firing pin marks alone.
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Ballistic Imaging more substantial indents on the cartridge head near the rim. Analysis of ejector marks can be made more difficult by the fact that the rim of the cartridge head is also where ammunition makers put their headstamp (brand identifier) and information on the size and caliber of the cartridge. These heavy-set alphanumeric characters are inscribed on the cartridge brass and—depending on where the ejector happens to hit—parts of the stamp may bleed into the ejector mark. In addition to the shape of the ejector mark and any individual scrapes or textures therein, ejector marks also serve the same important purpose as a firing pin drag mark: They provide a point of reference for proper orientation of cartridge cases relative to each other in comparison. Other Markings During the firing process, gas pressure works on all surfaces, forcing the material of the cartridge against the chamber of the weapon; particularly in semiautomatic weapons, other firearms parts are used to circulate ammunition through the weapon and eject spent casings. These actions and parts can lead to a host of marks on the cartridge case that—though not imaged using current techniques—are sometimes used by examiners studying matches between pieces of evidence. Chamber marks are parallel striated marks along the outer walls of the cartridge case, impressions from the scraping used to bore or ream the chamber (along with the rest of the barrel) from a solid piece of alloy. The extractor in a pistol that helps move a spent cartridge out of the chamber is typically a small arm that fits over the rim of the casing, holding it as the breech assembly slides backward. Accordingly, the extractor can leave marks where it makes contact, either on the edge of the rim of the cartridge head or on the neck separating the head from the main body. The slide that moves back and forth in semiautomatic pistols, allowing ejected casings to move away from the weapon, may leave a scuff mark on the edge of the cartridge head and a rough drag mark along the cartridge wall. As individual cartridges move from a magazine into chamber, a mark on the outer wall of the case may be caused by the magazine lip. 2–C.2 Bullet Markings Hatcher’s (1935:255) seminal text on firearms identification referred to “the fine ridges and grooves on the surface of the bullet, parallel to the rifling marks,” as “the most important individual characteristics which are used” in the field. These marks on the bullet—known as striations or striae—“are caused by its passage over surface irregularities and rough spots on the interior of the gun barrel that got there principally during
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Ballistic Imaging the machining operations of reaming the bore and rifling the grooves. Any such machining operation will leave the bore at least slightly rough, and each rough spot will leave a mark on the bullet during its passage through the bore.” The rifling carved into the barrel takes the form of grooves separated by raised areas, known as lands. These lands and grooves create corresponding engraved areas—dubbed land engraved areas and groove engraved areas (and commonly abbreviated as LEAs and GEAs)—on the bullet surface, separated by shoulders. The land engraved areas, being the part of the bullets that scrape against the raised lands on the barrel, are the principal areas of interest for observing striations. The pattern of land and groove engraved areas on recovered bullets can be used to determine basic information about the rifling characteristics of the gun that fired them, in order to identify a class of guns from which it came. Specifically, the number of lands is an important class characteristic, as is the direction of twist evident from a side view of the bullet. Bullets (and corresponding rifling characteristics) are commonly labeled by these two pieces of information—e.g., 5R for five lands and a right-hand twist. A recovered bullet can also be measured to suggest the caliber of the ammunition and weapon. However, this is not always possible—nor is a full analysis of striation marks—due to the condition of some bullets recovered from crime scenes (and victims). Bullets fired through weapons using polygonal rifling create special difficulties. Compared to conventional, square-edged rifling, polygonal rifling has key advantages: it reduces metal fouling, and it increases bullet velocity by reducing friction as the bullet passes through the barrel (Heard, 1997:123). However, the smoothness and subtlety of polygonal rifling can make it difficult to discern even gross features on recovered bullets—the shoulders defining lands and grooves—much less fine individual detail. Heard (1997:131) concludes that “generally speaking it is possible, although extremely difficult, to match bullets from polygonally rifled barrels.” 2–D THE MANUFACTURING OF FIREARMS AND AMMUNITION The underlying theory of firearms identification depends critically on manufacturing processes, positing that the tools used to form component parts wear with use so that each part may share the same gross features yet differ in microscopic (and, presumably, uniquely individual) ways. Manufacturing processes are also essential to consider in assessing the costs and benefits of wide-scale ballistic imaging or alternatives such as microstamping. Introducing stages to the process of producing firearms or ammunition—for example, systematic test-firing to produce exhibit cases, imaging of exhibits in large batches, or laser-etching a unique mark on the
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Ballistic Imaging base of a bullet—can have major impacts on the cost of production and, perhaps, the feasibility of compliance with proposed changes. We have already touched on some aspects of manufacturing in describing the anatomy of firearms and ammunition earlier in this chapter, and aspects of manufacture will arise in Chapter 3 as well (particularly in discussing challenging issues for firearms identification, generally). This section introduces basic issues but is not a comprehensive discussion. 2–D.1 Firearms The manufacturing of most guns is highly automated and generally efficient, and as many as 5 million new firearms (domestic and foreign) enter the U.S. market each year. Befitting its historical development, dating to Samuel Colt’s popularization of interchangeable parts and production line assemblies, the modern firearms industry remains one that is characterized by solid process control. That is, the process of mass-producing firearms is one that can be well partitioned: constituent parts of a new firearm can be drawn from large bins of fairly standardized parts and automatically fitted together with low yield loss, resulting in weapons of reasonably identical properties in terms of size, weight, and performance. Yet individual manufacturers differ on the exact steps used in machining and assembling firearms, and choices on the amount of filing or polishing to do on firing pins or whether to apply paint to the breech face can have an impact on the resulting toolmarks. In addition, some manufacturing techniques affect the type and quality of marks created in firing. Champod et al. (2003:307) argue that “machining marks made by grinding, filing and some other machining methods are random and hence we expect no repeatability between tools.” In comparison, “machining marks made by stamping, some cutting processes such as broaching, and some forging processes may be repeatable.” Various manufacturing techniques used by Lorcin Engineering drew interest in the 1990s, as firearms produced by the firm became more widely used in crimes;5 they serve as useful illustrative examples. Thompson (1996:95) found two Lorcin L9MM semiautomatic pistols, bought at the same time, that produced sufficiently similar breech face markings that a match could be made to either weapon on that mark alone; they could, however, be distinguished by sidewall and extractor marks. Similarly, Matty 5 In 2000, the Lorcin L380 semiautomatic pistol was the most traced firearm after recovery from juvenile possessors, and a Lorcin .25 caliber pistol ranked seventh. The L380 was also traced with high frequency after recovery from older offenders, ranked second among firearms recovered from 18–24-year-olds, and ranked third among firearms recovered from adults aged 25 and older (U.S. Bureau of Alcohol, Tobacco, and Firearms, 2002:15–16).
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Ballistic Imaging (1999:134) reports on a case where a search on a DRUGFIRE database—an initial competitor to the current Integrated Ballistics Identification System (IBIS) for ballistic imaging (described in Chapter 4)—suggested enough similarity to cause the physical evidence (both test-fired cartridge casings and the recovered Lorcin L9MM that produced them) to be retrieved from storage. On more detailed examination, “the breech face signatures were similar, but there was insufficient detail for an identification”; however, chamber and extractor marks failed to coincide at all. “The heavy black ‘paint’ that adhered to the breech face” was originally believed to be a cause of this phenomenon (Thompson, 1996:95).6 Ultimately, though, it was attributed to the fact that the breech faces for that model being formed by stamping, with no further grinding. In earlier Lorcin models, “the breechface area would become battered during firing as [a relatively soft alloy slide] hit the rim of a cartridge in the magazine as it fed the cartridge into the chamber”; this caused the breech face markings to be unstable and to change from firing to firing (Matty, 1999:135). Lorcin revised its process—in newer models, “a solid stamped steel insert is placed into a non-ferrous alloy slide”—but this stamped steel insert is prone to have marks that “can carry over from one steel insert to another” (Tulleners, 2001:3-4). (This phenomenon is an example of subclass carryover, discussed in fuller detail in Section 3–B.1.) More generally, Collins (1997:498) observed that “the bullets and casings of the [Lorcin] L380 [.380 caliber semiautomatic] pistol are easy to characterize. The bullets exhibit slippage7 and/or extremely shallow land impressions that often make even shoulder location difficult to determine,” and even “breech face marks are either non-existent or change from shot to shot.” Collins’ specific inquiry into the manufacturing pistol was based on attempting (unsuccessfully) to replicate crescent shaped marks observed in some firings, imprinted directly below the firing pin impression and believed to be caused by peening of the breech face surface under repeated firings. Another example of manufacturing processes that can directly affect the marks left by firearms and the ability to match them is the button rifling technique used by some manufacturers, notably Hi-Point (Roberge and Beauchamp, 2006:166): 6 A thick coat of black paint was also judged to be the probable cause of highly similar breech face marks produced by two different 45 ACP Haskell semiautomatic pistols; individual characteristics would emerge on the breech face marks for each gun with repeated firings, as the paint chipped and wore off (Tulleners, 2001:3-4). 7 “Slippage” means that a bullet does not fully grip the rifling on the barrel interior; hence, it can wobble and shift, rather than following the clear path of the rifling (and having marks carved into the side of the bullet as it passes through).
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Ballistic Imaging This process creates the grooves in the barrel by compressing rather than removing the excess material resulting in a relatively shallow barrel groove. Another distinct characteristic of the Hi-Point barrels is the metal tailings left along the shoulder of the groove. The combination of button rifling and metal tailings creates a relatively smooth barrel with very coarse shoulders. With each shot fired, all or part of the metal tailings break off changing the coarse stria on the fired bullet. The shallow rifling also allows a great deal of slippage to occur. Furthermore, the crowning8 of these barrels can add additional subclass characteristics. All newly manufactured firearms are required to bear a unique serial number, and this number may be stamped or etched on various parts of the firearm frame and assembly. However, guns with consecutive serial numbers are generally not consecutively manufactured in full. Production of firearms is typically an assembly line process, drawing various preconstructed parts from large bins for assembly into a finished weapon. Hence, two firearms that bear consecutive serial numbers may have rolled off the line in sequence, but their frames, barrels, firing pins, and so forth need not have been manufactured right after each other. There are some exceptions to this rule; for instance, Lardizabal (1995) found that consecutive serial numbers in a set of Hechler & Koch 9mm USP semiautomatic pistols meant that the slide for these weapons had in fact been consecutively manufactured.9 2–D.2 Ammunition Like firearms, ammunition cartridges are the result of numerous tooling and machining operations, and individual manufacturers vary in the specific techniques they use. It is standard practice for manufacturers to apply a head stamp, engraved on the rim of the cartridge head, to identify the manufacturers and perhaps the specific make of the ammunition; they may also use colored paints or other indicia to differentiate between specific makes and calibers. Ammunition manufacturers also vary in some postprocessing steps, such as the application of a lacquer sealant to the primer surface. “Primer sealants are routinely applied to centerfire cartridges to increase the power and reliability of the ammunition,” “placed at the junction between the primer and the primer cup [to] create a water and airtight 8 “Crowning” is a finishing step on the muzzle or discharge end of a barrel, rounding or grinding the mouth so that it is flush or recessed slightly and thus providing no obstacle to the bullet’s exit. 9 Lardizabal (1995:50) found that firings from a set of these pistols with similar serial numbers could not be distinguished from each other by any mark, and this “persistence of detail” continued through 250 firings. A pattern of striations was observed on the breech face itself, above the firing pin hole; this mark appeared to have been created after a chemical finishing process.
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Ballistic Imaging seal [and prevent] oil and other foreign matter from entering the cartridge.” The sealant also makes the cartridge resistant to moisture. However, while “most ammunition manufacturers limit the application of the sealant to the junction of the primer and primer cup,” some (primarily European) manufacturers “apply the sealant so that it extends across the entire surface of the primer.” The Czech-made Sellier and Bellot ammunition, in particular, is known for a red lacquer sealant over the entire primer (Hayes et al., 2004:139). The lacquer can act as a cushion, “absorb[ing] and dissipat[ing] a greater amount of energy” when involved in a collision (compared with metals), and consequently “reduc[ing] the amount of energy that reaches the metal surface of the primer” (Hayes et al., 2004:142). The specific techniques of a manufacturer can combine with more ornamental and postprocessing steps to leave distinctive marks on the cartridge. Box 2-1 reviews these nonfiring manufacturing marks—features that are present on the cartridge before firing and traces of which may endure after firing. In comparing exhibits, firearms examiners must compensate for the presence of these nonfiring marks, lest they lead to a false identification or exclusion. While many of these nonfiring marks are deliberate design choices, others arise inadvertently due to other steps in manufacture. Yborra and McClary (2004) report finding distinct striated markings near the edge of the primer surface on a batch of 115 grain Remington 9mm Luger ammunition. The marks appeared to be due to manufacturing and not firing: when a pair of casings was rotated so that identifying marks in the firing pin impression were in the same orientation, the extractor marks on the cartridges also lined up but the newly found striated marks on the primer surface were 90 degrees out of alignment. Remington managers indicated that they had never previously experienced such a phenomenon but suggested that a possible cause might be the way the primer is seated in the cartridge. Two separate punches drive the primer to its final position about 0.002 to 0.005 inches below the level of the cartridge head; “a misalignment or damage to one of these punches MAY have caused the observed [marks], and being machine-based, would be consistent” (Yborra and McClary, 2004:309). But no such defect could be found; nor could similar marks be detected on other boxes of ammunition from the same lot. The punches used in primer seating were also suspected of causing parallel markings near the edge of the primer on some Winchester 9mm ammunition (Flater, 2002:315); it was also suggested that the die used to flatten the surface of the primer cup could also have impressed such a mark.
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Ballistic Imaging BOX 2-1 Nonfiring Manufacturing Marks Nonfiring manufacturing marks on ammunition are features created by individual firms’ manufacturing processes. They are not defects, in that they do not diminish the ammunition’s performance or otherwise detract from the ammunition’s quality. However, they may be mistaken for textures or striations created by the firing of a gun or that may complicate the determination of a pattern match between exhibits. Amassing knowledge of these marks—and developing the skill to adjust for their presence—is an important part of the experience of a firearms examiner. Cataloging these nonfiring manufacturing marks, Tam (2001) suggests a rough typology based on their impact on the determination of a match between evidence: (1) marks that are not expected to cause a problem for identification (or exclusion); (2) marks that may cause problems but can be compensated for with some effort; and (3) marks that are problematic for comparison and difficult to analyze. In the first class, there are marks that would easily be overwritten by firing-related marks, as in extremely fine pre-existing parallel marks on the primer surface. Other marks—being relatively simple and known in advance—are not problematic because the examiner can mentally compensate for their presence (e.g., a V-shaped or other stamped mark on the primer surface used to indicate certain brands). Other marks that may fall into this category are those that are on areas of the cartridge not typically considered for ballistic imaging or routine analysis, such as unique marks on the rim of the cartridge. For the second class, manufacturing marks that may cause problems, Tam (2001) suggests that these features can be overcome by simple procedures. Marks in this class include thick striation-like parallel marks across the primer surface; these may obscure texture patterns in the breech face impression and may extend into the firing pin impression. Russian-made Wolf ammunition is well known for these marks, which have also been observed in other ammunition types. An IBIS image (using side light) of a fired round of Wolf ammunition is shown below; most of the visible horizontal parallel marks on the primer surface existed prior to firing. Side light IBIS image of fixed casing using Wolf ammunition; heavy horizontal lines are preexisting manufacturing marks.
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Ballistic Imaging Reitz (1975:103) observed “matchable striations on unfired primers of [exhibits from particular lots of] Winchester, .38 special cartridges.” These marks were attributed to a particular punch used during the primer seating process, which had not been produced to the same smoothness as is typically the norm. “These markings remained prevalent even after firing, which could be perilous to comparison examinations by unwary examiners.” Similarly, Robinson (1996:164) observed Russian-made ammunition with primers that, before firing, “had parallel marks like one might find as a result of breechface impressions.” Finding that “the marks continue around the curve of the primer into the sides which were not visible,” he concluded that “the only way that marks could have gotten there was by the rollers in the brass mill where the sheets of brass were made.” The third class of marks, those that are problematic for comparison, include ammunition types with existing distinct parallel and cross marks on the primer surface, making it difficult to discern which textural features were created by firing. Murray (2004:314) reports on toolmarks on the primer surface of Fiocchi .25 Auto ammunition whose cause is unknown; the manufacturer suggested that they might be attributed to a rare, imperfect configuration of the feeder during the process in which the primer is seated in the empty shell. The marks were problematic because they were not consistently prominent across the whole primer surface. When, as in the Wolf ammunition toolmarks, the markings span the whole primer, an examiner can compensate for them because they can be traced from the face of the primer into the pit of the firing pin impression. Maruoka (1994a; see also Maruoka and Ball, 1995) had previously noted parallel marks on the primer surface of some Fiocchi ammunition, but those marks did span the entire surface. But these inconsistent marks offer no such traceability, so that “differentiating these marks from breech face marks would be very difficult, if not impossible” (Murray, 2004:314). Some ammunition may also bear random marks on the rim of the cartridge that could be mistaken for ejector marks.