Forensic ballistics under fire

Interesting:

And in 2016, a government-appointed advisory group of leading scientists and engineers released a study that cited serious concerns about the underlying science of ballistic analysis. The study said the practice relies on circular logic and its “conclusions are subjective.”

“It’s basically a guess,” said Jon Loevy, an attorney representing Pursley’s case in civil court. “I’m sure there’s a lot of ballistics examiners that swear by the science, but that doesn’t make it scientific.”

Specifically, Loevy questioned the subjective nature of the analysis.

“Why do we believe science? It’s not just because it’s a guy who says ‘I’m a scientist’ giving it — it’s because you can replicate results with studies,” Loevy said. “Well, there’s no study that corroborates that this shell casing looks like that shell casing — it’s just some guy’s opinion.”

I know a fair amount about biometrics and it appears some of the same issues which trouble the use of fingerprints are also issues with forensic ballistic analysis. Verification is much better than identification. That is, the examiner can say that the odds that a given bullet (partial fingerprint) matches a given gun (person) with 99% certainty. But given a bullet (partial fingerprint) then finding the correct gun (person) out of the entire population of guns (people) the certainty might be only 0.1%.

Think of it this way: Suppose each gun stamped a number from 1 to 100 on each bullet fired. Given a random bullet and a random gun you could reject 99 guns out of 100 as not shooting the bullet. But if there are 100,000 guns, and the numbers are distributed evenly, 1000 of them could have shot the bullet.

The markings left on the bullets (and shell casings) don’t have sufficient persistent “resolution” to uniquely distinguish between all guns when the population of guns is very large.

9 thoughts on “Forensic ballistics under fire

  1. That’s not all of the problem. First, there is no scientific study that confirms the belief that markings are unique as you mention. But second, there is no confirmation of the reliability of a human examiner’s opinion regarding a “match”.

    Television shows have reinforced the belief that forensics is “science” but it’s really propaganda for practices that have little basic scientific foundation.

  2. But how will the crime shows be able to state that the boolit recovered from the victim was fired from a Glock Model 26 with a FDE frame, before the murder weapon is recovered. I know, “micro stamping” and serial numbers on boolits.

    Unfortunately fiction does not translate to reality, as much as some lawmakers, prosecutors, and jurors want it. With mass produced firearms, and interchangeable parts, forensic ballistics can be useful but not as definitive as believed in pinpointing a specific firearm.

  3. The ability to examine a fired bullet and identify the type of firearm, rather than the individual firearm, from which it came is important. If Joe Suspect is arrested with a .380ACP Glock in his possession, and the person he is suspected of shooting has a 158 grain bullet in him with 5 rifling marks on it, Joe’s gun is NOT a match to the bullet.
    That alone is useful information.

    Likewise, if Joe is arrested carrying a Glock 9mm and the suspect has been shot with a 115 grain bullet with six rifling marks, Joe might just be your guy.

    At that point, further comparison of the bullet from Mr. Victim and a bullet test-fired from Joe’s Glock might show enough matching attributes unique to that gun that calling them matched is honest, especially if they are not generally found in other Glocks (like a scratch caused by a chipped extractor).

    A good defense attorney might of course point out that everybody owns a Glock, and every Glock makes lots of marks matching the found bullet, and that in fact there are mismatched marks on the found bullet versus the test bullet. This is fair, and honest, and up to the jury to decide how important or unimportant to deciding Joe is guilty or not.

  4. At that point, further comparison of the bullet from Mr. Victim and a bullet test-fired from Joe’s Glock might show enough matching attributes unique to that gun

    You see that’s the part that is the real debate. Is there any scientific basis, not just made up stories, to the claim that any particular technician can “match” any “attributes”. And what is the actual confidence in such a conclusion?

    As an example, everyone knows that latent fingerprint analysis is reliable right? Uh, maybe …. https://www.aaas.org/news/fingerprint-source-identity-lacks-scientific-basis-legal-certainty

    Hair comparison? That’s great forensic ‘science’ right?

    https://phys.org/news/2015-04-doj-fbi-acknowledge-flawed-testimony.html

  5. A good defense attorney might of course point out that everybody owns a Glock, and every Glock makes lots of marks matching the found bullet, and that in fact there are mismatched marks on the found bullet versus the test bullet.

    All true and good, but….the number of attorneys who have sufficient comprehension of the facts necessary to elucidate such is pretty small, and the number of jurors capable of understanding the explanation smaller yet. Indoctrination works, and The Dozen have been steeped in CSI and NCIS long enough to expect magic, and vote for it.

  6. The only time I ever got on a jury, the alleged crime was drug sales, cocaine, in Baltimore. The first thing every juror saw as we walked into the jury room for deliberations was a Sports Illustrated with Len Bias on the cover. Purposeful or just coincidence? Go figure. The poor sap was guilty as hell, anyway.

  7. The only time I ever got on a jury, the alleged crime was drug sales, cocaine, in Baltimore. The first thing every juror saw as we walked into the jury room for deliberations was a Sports Illustrated with Len Bias on the cover. Purposeful or just coincidence? Go figure. The poor sap was guilty as hell, anyway.

    As to analysis of marks, I’ve done some of that (not forensically, for business purposes). A quick microscope image comparison is amazingly useful for picking out similarities and differences in solid objects.

    • Yup. Very interesting. I’ll be more interested to know what the numbers look like with the number of guns is 21,000 instead of 21.

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