the prominent markings on cartridge cases—has not been documented, there may be structural or design features that may cause a similar effect. The Thompson (1996) investigation of highly similar marks resulting from different Lorcin L9 9mm pistols (see Section 2–D.1) included some firings from guns right off the manufacturing line, with no previous proof firing. It was noted that the “breech face impression … change considerably shot-to-shot [in early firings] due to the paint wearing/chipping off.”
As detailed in Chapter 1, it is not the function of this committee to assess the general validity of firearms identification and toolmark examination nor their admissibility in court proceedings. The discussion in this chapter on the nature of toolmark evidence and the context in which it is applied, as well as an overview of existing research among firearms examiners on the uniqueness and reproducibility of toolmarks, is presented to frame the discussion in the rest of this report. For instance, understanding situations that may pose particular challenges for associating two images of evidence requires some knowledge of situations that are generally known to be complex in the field; likewise, recommendations for the setup and maintenance of any ballistic imaging system that would do harm to the maintenance of clear chain of custody—so important in the legal context of toolmark evidence—would be ill-advised. However, as we also note in Chapter 1, we understand that some readers may try to infer a position—a leaning, one way or the other—based on the preceding analysis.
Accordingly, we believe it important to make the committee’s finding clear and unambiguous:
Finding: The validity of the fundamental assumptions of uniqueness and reproducibility of firearms-related toolmarks has not yet been fully demonstrated.
There is one baseline level of credibility, however, that must be demonstrated lest any discussion of ballistic imaging be rendered moot—namely, that there is at least some “signal” that may be detected. In other words, the creation of toolmarks must not be so random and volatile that there is no reason to believe that any similar and matchable marks exist on two exhibits fired from the same gun. The existing research, and the field’s general acceptance in legal proceedings for several decades, is more than adequate testimony to that baseline level. Beyond that level, we neither endorse nor oppose the fundamental assumptions. Our review in this chapter is not—and is not meant to be—a full weighing of evidence for or against