exhibits. However, the breech face marks were “not useful for matching any of the 2500 cartridges to the weapon” because the presence and clarity of these marks were inconsistent. 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. (1988) completed a rarer study of actual shot-to-shot variability for a string of 100 firings. Specifically, they collected bullets and cartridge casings from each of the 100 test firings, divided between four different combinations of gun brand and caliber. Each mark—land areas on the bullets and breech face and firing pin marks on the cartridges—was examined separately by examiners to assess the quantity of unique, individualizing lines or features. Each mark was then assigned an identifiability score of 1, 2, or 3; 1 indicated few or no identifiable features and 3 indicated sufficient features to support an identification based on that mark alone. Each bullet and casing was then assigned a score corresponding to the maximum identifiability score of its constituent marks. The analysis stopped short of true shot-to-shot comparison of features, although this was done for some sequences of well-marked (identifiability score 3) bullets.
For centerfire cartridges, they observed an approximately 10–50–40 percent division across identifiability grades 1, 2, and 3, respectively. Breech face marks generally showed higher identifiability scores than firing pin marks, which Uchiyama et al. (1988:378) attribute to the greater surface area of the breech face marks; however, for firings from a .38 Special Smith & Wesson (for which “the surface of the breech face is rather smooth and the firing pin is large and has irregular markings”), firing pin marks outperformed breech face. Identifiability scores dropped markedly for rimfire cartridges, with only about 5 percent having score 3 and 35 percent score 1; none of the breech face markings for rimfire casings was judged to have identifiability level 3. For bullets, identifiability level 3 ranged from 20 to 60 percent across the different gun/caliber combinations, and Uchiyama et al. (1988) judged that none of the test-fired bullets fell into identifiability level 1.
Blackwell and Framan (1980:16) cite discussions with an examiner at the Los Angeles Police Department in suggesting a “reproducibility spectrum,” a continuum of manufacturing standards and firearms user practices that can affect the reproducibility of individual marks. At one extreme are police officers, for whom a firearm is such a vital part of their equipment