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.”