Ballistic Imaging (2008) / Chapter Skim
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3 Firearms Identification and the Use of Ballistics Evidence
3 Firearms Identification and the Use of Ballistics Evidence
Pages 53-88
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From page 53... ...
The circumstances of firing a weapon, the manufacture of firearms and ammunition, and measures taken (or not taken) by firearms users can all pose various complications to the identification and linking of ballistics evidence; we review some of these issues in Section 3–D.
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From page 54... ...
Firearms toolmark evidence differs from DNA and fingerprint evidence due to their basic point of reference: the former links to a particular firearm while the latter two link to a particular person. Links between pieces of ballistics evidence can point to a common gun from which exhibits were fired, but not necessarily to the same person pulling the trigger.
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From page 55... ...
3–B Traditional Firearms Identification Smith (2004:130) succinctly summarized the basic task of a firearms examiner in making an identification between pieces of evidence: Before a microscopic comparison begins, a foundation is built by measur ing and comparing available class characteristics, such as General Rifling Characteristics (GRCs)
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From page 56... ...
on "individualization" versus "identification.") The phrasing of "individualization" as the act of associating an object urther F information on current practice in firearms identification and images connected with that work are available through resources at the Association of Firearm and Tool Mark Examiners (http://www.afte.org)
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From page 57... ...
. The basic approach to identification in forensic science developed into the concept of class characteristics and individual characteristics.
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From page 58... ...
• Can arise from a source which changes over time Caution should be exercised in distinguishing subclass characteristics from individual characteristics. As a middle ground between class and individual characteristics, subclass characteristics covered marks or features that -- arising from specific manufacturing techniques, or flaws in said techniques -- could induce similar marks on pieces of evidence even though they originated from different ectangular R firing pin marks were a telltale sign of Glock pistols "prior to the introduction of Smith & Wesson's Sigma Series, Model SW40F, semi-automatic pistol," the design of which is very similar to Glock standards.
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From page 59... ...
The examiner should be conservative when reporting the significance of these observations. The following represents a spectrum of statements: A. IDENTIFICATION: Agreement of a combination of individual characteristics and all discernible class characteristics where the extent of agreement ex ceeds that which can occur in the comparison of toolmarks made by different tools and is consistent with the agreement demonstrated by toolmarks known to have been produced by the same tool.
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From page 60... ...
C. ELIMINATION: Significant disagreement of discernible class characteristics and/or individual characteristics. D. UNSUITABLE: Unsuitable for comparison.
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From page 61... ...
observed that the particular tooling used on some Smith & Wesson firing pins results in a pattern of concentric rings that is re peatable in consecutively tooled pins. This pattern appears in the firing pin impres sion, meaning that examiners need to downweight the ring patterns and focus on other individual features to make correct matches on the firing pin impression.
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From page 62... ...
• 1907: Cartridge case evidence was examined as part of the investigation of the 1906 Brownsville, Texas, riots in which black soldiers allegedly fired upon a white crowd in retaliation against racial slurs. Thirty-nine cartridges were recov ered; all but six were matched to four rifles, based on examination of enlarged photographs of firing pin impressions.
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From page 63... ...
Foreshadowing some contemporary claims about the capability of ballistic imaging systems, Burrard (1962:131–132) lamented: The most fantastic claims have been put forward for, and the most r idiculous descriptions of, [the comparison microscope]
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From page 64... ...
In recent years, as the increased workload of state and local law enforcement forensic laboratories has created a shortage of firearms examiners, p lacing increased attention on the training and recruitment of examiners. Noting that "there is no college degree a firearms examiner can pursue to reach true proficiency in firearms identification," and that the core of training activities consists of "extended on-the-job training," the Bureau of Alcohol, Tobacco, and Firearms (ATF)
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From page 65... ...
relates that this approach has developed and emerged to the point that a "fray appears to have developed within the discipline," pitting "the old school tradition of ‘pattern matching' versus the new school of ‘line counters.'" Biasotti's (1959:34) work was motivated by what he described as "an almost complete lack of factual and statistical data pertaining to the problem of establishing identity in the field of firearms identification." Accordingly, he sought to "conduct a direct statistical count of the elements which actually form the basis of the identity; e.g., the individual characteristics;" he did so using a set of test firings from .38 Special Smith & Wesson r evolvers.
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From page 66... ...
found compelling, though, was evidence of runs or series of consecutive matching striations: It should be obvious that consecutiveness; viz., the compounding of a n umber of individual characteristics, is the very basis of all identities. When individual characteristics are grouped or related by the criteria of consecutiveness, which is a simplified means of expressing a correspon dence of contour, the chance occurrence of even a very small number of consecutive matching lines (e.g., more than 3 or 4)
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From page 67... ...
Consequently, firearms examiners may often express their findings in bold absolutes -- matches made to the same gun, to the exclusion of all other firearms in the world -- yet they tend to be conservative in reaching their o pinions. If a firearms examiner is impeached through the voir dire process, his or her ability to testify in other cases can be severely affected; being associated with an error or misidentification can tarnish reputations.
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From page 68... ...
Like other branches of forensic science, firearms and toolmark identification has had to grapple with the question of how well the field fits emerging standards for scientific evidence in legal proceedings.
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From page 69... ...
Branches of forensic science like toolmark evaluation are increasingly concerned with how their fields fit either standard, cognizant that the p recedent of a single ruling of inadmissibility could jeopardize future proceedings. That question has grown ever greater with the Florida Supreme Court's December 2001 ruling in Ramirez v.
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From page 70... ...
in state and local law enforcement laboratories as adjuncts to their regular casework. The review articles attempt to piece together major themes from decades of such studies, most having been published in the AFTE Journal but also in other forensic science publications.
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From page 71... ...
In addition to being T a weak proxy for consecutive manufacture, sequential serial numbers may also suggest large batch purchases of firearms, such as the deployment of new duty firearms among police o fficers. In our analyses of the New York CoBIS reference ballistic image database, we encounter runs of consecutively serial-numbered firearms with similar IBIS comparison scores.
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From page 72... ...
Bonfanti and De Kinder found it hard to classify these problematic cases by manufacturing technique, but offered as "a probable solution to this problem" a common theme that recurred in several of the studies. Specifically, individual examiners noted remarkable similarities on some of the marks but found that they could make identifications using the full range of markings on the cartridge cases, not just the firing pin impression or breech face mark.
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From page 73... ...
From the 10 studies, Bonfanti and De Kinder (1999a:319, 321) conclude that the major cartridge case markings are relatively less vulnerable to change: "no substantial change in characteristics left by the breech face of the weapon can be discerned" over repeated firings; firing pin impressions and extractor marks are subject to "slight variations," while "in one study the ejector marks were seen to vary more strongly." Bullets, by comparison, showed more dramatic effects due to wear; this is not greatly surprising given that a firearm's barrel is subject to the most fouling (deposition of firing residue)
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From page 74... ...
Firing pin impressions proved more useful although, "after round 70, the nose of the firing pin was beginning to lose its imperfections resulting in a small flattened circular area at the bottom of the marks. Although firing pin drag marks were not consistently produced throughout the 2500 round firing sequence, the last 200 rounds did contain sufficient stria to match the cartridges to the weapon." Meanwhile, the value of ejector marks to making identifications "was enhanced" over repeated firing; the size and depth of the ejector mark increased "after firing 600 rounds." In comparison with these longevity studies, Uchiyama et al.
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From page 75... ...
Likewise, Tulleners (2001:3-2) wrote that, "for automated imaging, the only areas used are the firing pin impressions, breech face marks, and ejector marks" because "these are the marks that are typically repeatable." However, "in most cases the firing pin may not leave sufficient detail for analysis and most examiners rely on the breech face marks" to make identifications (Tulleners, 2001:3-2)
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From page 76... ...
so that bullets may be recovered with no damage other than those left by the firing process; similarly, cartridge casings may be quickly and safely retrieved following controlled test firings. A basic, fundamental challenge of firearms identification is that crime scenes are not necessarily such controlled settings.
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From page 77... ...
observed that, "periodically, firearms examiners and [IBIS] technicians receive requests to examine firearms that have been oxidized (rusted)
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From page 78... ...
Corrosion is a concern not only for the internal parts of firearms, but also for recovered ballistics evidence. Though it is certainly ideal to process and collect evidence from crime scenes rapidly, such is not always possible.
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From page 79... ...
The evidence casings did not appear to match test firings from the firearm; the latter all showed much greater levels of flowback around the firing pin impression. Subsequent analysis determined that an attempt had been made to alter the firearm by drilling the entire length of the pistol's barrel; however, the person attempting the modification apparently inserted the drill bit into the firing pin aperture, widening the hole but also damaging the tip of the firing pin.
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From page 80... ...
It is certain that the similar markings visible now would become less prominent after being subjected to normal firing, cleaning, and wear." Even though all the test firings Lutz examined were among the first 12 uses of the barrel, "sufficient matching individual striae were noted on the bullets to enable the examiners to easily identify the barrel of origin for each of the bullets. Similarities in class characteristics were noted; however, microscopic comparison of the bullets revealed that each barrel had caused different markings to such an extent that each land and groove impression on each of the bullets had a great number of individual identifying striae." Though a prominent settle-in effect for the breech face or firing pin --
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From page 81... ...
For instance, understanding situations that may pose particular challenges for associating two images of evidence requires some knowledge of situations that are generally known to be complex in the field; likewise, recommendations for the setup and maintenance of any ballistic imaging system that would do harm to the maintenance of clear chain of custody -- so important in the legal context of toolmark evidence -- would be ill-advised. However, as we also note in Chapter 1, we understand that some readers may try to infer a position -- a leaning, one way or the other -- based on the preceding analysis.
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From page 82... ...
For example, in a large number of test firings, how comparable is the quality of breech face marking with firing pin impressions, and how do those compare with the clarity of striations etched on bullets? Fully assessing the assumptions underlying firearms identification would require careful attention to statistical experimental design issues, as well as intensive work on the underlying physics, engineering, and metallurgy of firearms, but is essential to the long-term viability of this type of forensic evidence.
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From page 83... ...
The defense also challenged the validity of any identification because the examiner replaced -- "among other parts" -- "the firing pin, recoil spring, barrel, and trigger lever (but, significantly, not the breech face) " of the gun in order to get it back into firing condition.
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From page 84... ...
But, in view of the thousands of criminal defendants who have had an incentive to challenge firearms examiners' conclusions, it is significant that defendants cite no false positive identification used against a criminal defendant in any American jurisdiction.
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From page 85... ...
3–F Role of Imaging in Firearms Identification In the next several chapters, we explore the current state of ballistic imaging technology. As context for this discussion, we note that imaging and photography have a long and somewhat controversial history in traditional firearms identification.
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From page 86... ...
. Prior to the use of automated ballistic imaging, law enforcement agencies such as the Los Angeles Police Department (LAPD)
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From page 87... ...
using conventional microscopes," it "was very complicated and time-consuming"; up to 3 firearms examiners at a time worked continuously on the comparisons. Ultimately, test-fired casings from the 3,704th pistol in sequence matched the casings from the second and third murders; this led investigators to an officer in Stuttgart who was ultimately found dead in southern Italy, having also murdered his wife and sons.
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