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3–C UNIQUENESS, REPRODUCIBILITY, AND PERMANENCE OFMARKINGS IN TRADITIONAL FIREARMS IDENTIFICATION
In recent years, several review articles have summarized the findings of individual studies on the basic principles of firearms and toolmarks—the uniqueness, reproducibility, and permanence of individual characteristics, as seen by trained examiners using comparison microscopy. Most of these studies are limited in scale and have been conducted by firearms examiners (and examiners in training) 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. Nichols (1997, 2004) contributes a two-part narrative with the goal of characterizing the state of the field. Bonfanti and De Kinder (1999a, 1999b) review a broad array of experimental studies—on the influence of manufacturing techniques (e.g., consecutive tooling) and the endurance of marks over repeated firings, respectively—distilling the studies as entries in extensive summary tables. We draw from these review papers in this section and excerpt additional detail from individual studies, as appropriate.
3–C.1 Uniqueness of Markings
A fundamental assumption in firearms identification is that individual firearms vary microscopically in ways that leave unique markings on bullet and cartridge case evidence. Accordingly, the “gold standard” for demonstrating the uniqueness of toolmarks in ballistics evidence would be sets of firearms that are consecutively manufactured—that is, more than one gun where every constituent part is subjected to identical tooling and machining operations (e.g., minimizing the wear on cutting tools between different pieces). This is difficult to achieve in practice given the assembly-line nature of firearms manufacturing, where individual parts may be made in advance (and not necessarily at the same facility) and are drawn from bins prior to assembly. Due to the time and manufacturer cooperation needed to ensure consecutive manufacture, those studies that have been done—as summarized by Bonfanti and De Kinder (1999a)—deal with small numbers, typically less than 15.8 It is also typically the case that only one part (or
One entry in the tables in Bonfanti and De Kinder (1999a) stands out: amidst studies with small numbers of firearms with some level of consecutively manufactured parts, Grooß (1995) is reported as having examined 3,704 Walther P5 pistols. The entry is misleading in its placement in the tables—Grooß makes no claim of consecutive manufacturing or serial numbering—but the study is still interesting as a case study of a wide-scale search of evidence using conventional microscopy rather than being assisted by imaging methodology; see footnote 17 in this chapter.