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increasingly compared with one of their sister branches: the analysis of deoxyribonucleic acid (DNA) markers and identification based on those markers. As we note in Section 1–C, the term “ballistic fingerprinting” has also come into common (albeit somewhat inaccurate) usage to describe some features of firearms identification, suggesting a comparison with fingerprint evidence.
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. A potential match suggested by a national reference ballistic image database could suggest a link from a piece of crime scene evidence to an original firearm point of sale, but that link is at least doubly indirect: the link is only to the location of the transaction and not immediately to the firearm’s purchaser, and subsequent identification of the purchaser does not necessarily mean that the purchaser still possesses the gun (or fired the shot in a crime). However, it is important to consider that—alone, absent any other evidence or knowledge of circumstances—even person-specific fingerprint and DNA evidence is necessarily one step removed and indirect. It is possible for fingerprint or DNA evidence to be present and retrievable at crime scenes without its source person having been the crime’s perpetrator. Forensic evidence can be used—in combination with other investigative findings—to develop a theory of what transpired at the scene and who may have committed the crime, but the link to any specific person from the ballistics evidence alone is necessarily indirect.
Toolmark evidence and DNA evidence are markedly different in another crucial respect, which is the subjectivity inherent in the analysis. Firearms identification ultimately comes down to a subjective assessment—specifically, a subjective probability statement (although practitioners often render these as absolute statements). Firearms examiners observe concrete, objective phenomena, but—as Thornton and Peterson (2002:24–25) observe, “there is an incredible amount of difficulty attached to the development of a statistical basis for evidence evaluation” in forensic science fields like firearms examination:
Behind every opinion rendered by a forensic scientist there is a statistical basis. We may not know what that basis is, and we may have no feasible means of developing an understanding of that basis, but it is futile to deny that one exists…. The most common and coherent theory of forensic identification is that where there is a high degree of variation among attributes (of toolmark striations, writing, friction ridges on skin, and so on), then where a “match” is observed the probability that the match is