Conclusion: A national reference ballistic image database of all new and imported guns is not advisable at this time.
Three lines of reasoning have particular salience for this conclusion. The first has to do with the general use and role of ballistic imaging technology. The current technology in use for automated toolmark comparison, based on two-dimensional greyscale images, can be useful for gross categorization and sorting of large quantities of evidence. However, it appears to be less reliable for distinguishing extremely fine individual marks as is necessary to make successful matches in RBIDs, where large numbers of exhibits on file would share gross class and subclass characteristics.
Throughout the report, and particularly in Chapter 4, we make it clear that we view ballistic imaging as a form of computer-assisted firearms identification and advise against practices—like overreliance on “top 10” comparisons—that impute to ballistic imaging an unwarranted level of precision for identifying matches. The temptation to expect too much from a national RBID—to expect “hits,” and investigative leads to points of sale, with high frequency—is misguided given that the event of a single, particular new gun being used in committing a crime is relatively rare. The difficulty in achieving matches in an RBID is compounded by the gross sameness—in class and subclass characteristics—of large segments of the database exhibits. Ballistic imaging can be an effective tool for screening and filtering, and can be 70–95 percent successful in finding same-gun matches using cartridge case markings, as Nennstiel and Rahm (2006b:28) concluded. This is very good performance, but De Kinder et al. (2004) compellingly demonstrate that this performance can degrade in databases flooded with same-class-characteristic images; we saw much the same thing in our limited work entering exhibits in the New York CoBIS database (described in Chapter 8).
The second salient argument concerns the capacity of ballistic imaging systems to distinguish true matches from nonmatches, as described in Section 9–B.3 and Chapter 8: Basic probability calculations, under reasonable assumptions, suggest that the process of identifying a subset of possible matches, that contains the true match with a specified level of certainty, depends critically on as-yet-underived measures of similarity between and within gun type. The process may return too large a subset of candidates to be practically useful for investigative purposes.
We emphasize that we do not frame this argument strictly as a “breakdown” or massive degradation in matching capability with database size. Pure reliance on a numeric breakdown argument maligns all forms of ballistic imaging—a national RBID most immediately, due to the large