They put a lot of faith in firing pin and chamber marks.
The only way to make that work at all is to make it illegal to perform any work on a gun without retesting it at state bllistic ID labs each time.
Aw, man, don't give 'em any ideas.
It doesn't have to be accurate or useful, as long as it serves in court to felonize another gun owner.
Yes, juries are impressed by lab techs high magnification photos and the ubiquitous phrase of the professional technocrat dissembler "this is consistent with that". Of course, any inconsistencies found are often over-looked or not reported. Lab techs aren't paid to generate evidence favorable to the defense. Few defendents can afford independent reviews of forensic evidence, and of course, any exculpatory Brady Material is easily left forgotten in the back of some cabinet or desk drawer. Prosecutors are required to disclose exculpatory evidence, but if they are never informed of the possibility because they are filtered out by helpful lab techs, then everybody is happy.
Actually none at all. This is simply a means of raising the barrier for gun cost and hence ownership. It's a cynical exercise in trying to kill a God given, Constitutionally protected right by regulation. New Yorkers - what a bunch of good little communists.