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Comment by sgarland | Hacker News Reader

How is ballistics forensics unreliable? Genuinely curious, I’ve never heard that.

It’s pretty much just complete non-replicable BS. The claim is that the rifling on gun barrels differs from gun to gun, and that they can examine grooves made in the bullets by the rifling and compare them to other rounds shot through the same barrel and find a match. It doesn’t stand up to independent scrutiny, and I would imagine that with modern machining the gun to gun differences in rifling on the same mass produced guns are minimal. Perhaps there are differences in fowling in well-used guns, but the notion that they’re gonna match a flattened bullet to a specific barrel is ridiculous. Basically a few kooks across the country wear white coats and claim that they can do this “science” and prosecutions pay them to be expert witnesses. Juries eat it up. It’s really nuts.

The only legitimate analysis they’re doing is ammo brand matching. “Hey, there was a Sellier & Bellot 380 Auto casing at the murder scene. The suspect had S&B 380 in his gun safe. We got him.”

You should see the games they play with statistics.

Find some 1 in 1000 match. Find another 1 in 1000 match. Claim they're independent, so the chances of both of these are 1 in a million.

What they don't say is that there are 5000 things you can test that each have a 1 in 1000 chance, so you should expect to find ~5 at random if you test them all. The 1 in a million chance is if you choose 2 of the 5000 at random, test only those and they both match; not if you systematically run tests that only notify you when they find one of the matches.

This reminds me of another bit of psudeo-science bullshit, claims that bullet wounds can be matched to close bullet size, like saying someone was hit by a 9mm vs .45 cal vs .40 cal if there is no bullet to recover.

There's really no way to relate rifling/etc to a specific gun. Unless it was forged in some bespoke process that can also be accessed and replicated the rifling pattern will look like 100,000 different guns. Other analysis is also bunk (blood spatter, gunpowder stuff, etc). Ammo matching can provide circumstantial evidence but is likely defeated by just shutting your mouth.

Most criminals subject to this are probably caught because they loaded the magazine without gloves and left a fingerprint on the casing that gets completely etched into it on firing. Or, you know, police do actual police work and pressure the right people into narc'ing on the suspect.

Radley Balko has a great overview on the fundamental flaws with ballistics forensics[1]. The tl;dr is that the core claim of ballistics forensic analysis, that it is possible to reliably differentiate between two models of the same gun using just casings and shells fired from those guns, has never been proven to be valid.

[1]: https://radleybalko.substack.com/p/devil-in-the-grooves-the-...

Great read, had no idea it was so incredibly bad! Definitely like pseudo-science, with extremely poor reproducibility. And a lot of dishonest massaging of words to make it seem much more certain than it is, benefiting the prosecution only. Imagine being innocent and having an “expert” take the stand and say it’s a “practical impossibility” that the bullet was fired from any other gun. The analogy with the tarot cards seems appropriate.

Seems like markings can be exculpatory at best, for instance that casings or bullets clearly don’t match, but that the “unique markings” of an individual barrel is complete BS, especially with precision mass production of modern firearms. But in practice, it seems to rarely exonerate suspects, even if the evidence exists.

Some of it (e.g. what bullet type) is real, but angles and distances are largely impossible to reliably.

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