Firearms and Toolmarks Identification

William A. Tobin & Peter J. Blau, Hypothesis Testing of the Critical Underlying Premise of Discernible Uniqueness in Firearms-Toolmarks Forensic Practice (Dec. 6, 2012) (footnotes omitted):

Mainstream scientists are likely in agreement that, at some level above the subatomic, every object in the universe is probably unique. Nevertheless, it is argued in the scholarly literature, and correctly in the authors’ opinions, that uniqueness is “largely irrelevant” to the forensic identification practices, which would include firearms identification. The more seminal issue for forensic practice, however, is that of discernible uniqueness. Similar to the familiar riddle of a tree falling in a forest with no one around, if uniqueness does, in fact, exist, what is the probative value if firearms examiners cannot discern it? But to discern uniqueness at some level, it is axiomatic that two conditions must exist: (1) some criteria, indicia, or “parameters of detection” for uniqueness, and (2) rules of application for those indicia to discern “same” from “different.” An exhaustive review of the domain literature reveals no such criteria. Thus, there is no apparent official or scientifically acceptable protocol for distinguishing ‘same’ from ‘different’. 
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