that they “[develop] pride and interest in maintaining it in superior working condition.” However, this “constant attention to cleaning and polishing” serves to “completely obliterate many, if not all, of the original identifying imperfections in the bore.” “Many officers mount their pistols in a drill press and actually wire-brush the bore to the point that individual features originally in the bore are removed, and new ones created.” Military small arms represent the other end of the spectrum:
These firearms, because of the nature of mass production, frequently retain their identifying characteristics for the life of the firearm. During manufacture, the bores are not carefully polished or lead lapped and the small blemishes that cause striations are retained virtually unchanged. Subsequent perfunctory or even diligent cleaning frequently does not obliterate them.
“The firearms that most frequently find their way into the firearms identification laboratory” fall in between these two extremes.
From the available studies of the reproducibility and endurance of firearms toolmarks, it is difficult to conclude that any of the markings are inherently more reliable than others. De Kinder (2002a:200) argued that “the breech face of the firearm seems to provide the most stable trace on the components of the fired round” and hence suggested that it might be the primary mark collected in a large-scale reference ballistic image database (RBID). However, he also observed that “important similarities were seen between marks left by the breech faces of subsequently manufactured firearms”—that is, the breech face may be prone to subclass carry-over effects—and so an RBID would likely have to include other marks like the firing pin. 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).
Over the course of years of analysis, forensic firearms examiners encounter numerous situations that complicate identification. These may be deliberate measures taken by the shooter—countermeasures to identification—but they may also be mechanical problems in firing. Smith (1971) cataloged 24 such situations, which he dubbed “jokers” in the field of firearms identification. He suggested that the top 10 such complications, “in the approximate order of frequency of occurrence,” are: