In some recent discussions related to ballistics evidence I recalled this post about the National Research Council saying ballistics databases aren't that effective.
Well, as part of those discussions I dug up the press release from the National Research Council. It supports the earlier story, but has some interesting passages as well.
The report notes that the fundamental assumption underlying forensic firearms identification – that every gun leaves microscopic marks on bullets and cartridge cases that are unique to that weapon and remain the same over repeated firings – has not yet been fully demonstrated scientifically. More research would be needed to prove that firearms identification rests on firmer scientific footing, said the committee that wrote the report.
Claims of Certainty About 'Matches' Without Firm Grounding
The report does not assess the admissibility of firearm toolmark evidence in legal proceedings, since making such a determination was not part of the committee's charge. However, it cautions that the statement commonly made by firearms examiners that "matches" of ballistic evidence identify a particular source gun "to the exclusion of all other firearms" should be avoided. There is currently no statistical justification for such a statement, and it is inconsistent with the element of subjectivity inherent in any firearms examiner's assessment of a match.
If firearms identification is to rest on firmer scientific ground, more research would need to assess the fundamental assumption that toolmarks are unique and remain recognizable over time, despite repeated firings. Such research should include a program of experiments covering a full range of factors that may degrade a gun's toolmarks, as well as factors that might cause different guns to generate similar toolmarks. Intensive work is also needed on the underlying physics, engineering, and metallurgy of firearms, in order to better understand the mechanisms that form toolmarks as a weapon is fired.
So, there's no evidence that ballistic matches are viable evidence over time.
I wonder how many convictions have been obtained nation wide on ballistic evidence, and how many of those convictions are only the result of faulty science.
The full report is available National Academies Press. In case you're wondering, no that's not a paid referral.