Relodr,
If you assume TV script writers' imaginations and plot lines are "science", then there's no discussion......
OTOH, if you want to discuss the vaguries of firearms forensics, then there's lots of topics/room, as previous posts have noted. Firearms forensices is out of my ken. But the manufacture of precision machined parts isn't. Not too much difference twixt the two, IMO.
IME, with modern tooling/techniques, the 'difference' between individual units in a production run with the same tooling is going to be minimal. The 'differences' may be so small or frangible as to be lost in "hysterisis". I.e. the inevitible 'noise' that creeps into any mensuration protocol. (Remember that word, "protocol", BTW.) All tooling wears in use. Probably, if one had the pure products of that tooling in sequential order, one could describe the wear changes and extent. But that doesn't work with any firearm.
First, most, if not all are test fired at the factory, then cleaned and inspected/adjusted. That process does its part in changing the 'original' markings AWA add its own distinctive 'signature'. Now the gun is sold and used. Each shot contributes its share to changing the barrel/land/groove characteristics. As does each cleaning. And then there's the issue of 1st shot/last shot. A cold barrel will likely produced markings slightly different from a hot one. IOW, the more you know, the more questions you have and the more 'uncertainity' creeps into the testing equation. And here's where the devil "protocol" rears his head.
Our national "experts" BATFE, have never established any testing protocols for firearms. Tends to "confuse" their allegations, I suspect. OTOH, the FBI has established many. Most are probably useful benchmarks, but science and techniques change; often far more rapidly than the forensic analysts and law recognizes. And, just as modern statistical analysis has demonstrated with "gold standard; fingerprints", not all traditional firearms forensic assumptions and techniques are as "bulletproof" as once thought. But the law still makes those assumptions in many cases, so the "salesman's job" is a cogent criticism. >MW