I was watching the ncis show? I think thats right, they were looking at a shell casing, gal with pig tails says they can ID the gun thru the serial number embedded in the firing pin, leaves a ID on the primer.......
I would suspect that the majority of Hollywood writers don't have much of a clue about anything mechanical/real world, whether it be firearm related, cars etc.
They are always identifying what kind of car/truck based upon tread pattern, as if someone doesn't wear out/change/buy different tires. How many of us bring a new vehicle home and immediately get rid of the tires let out to the lowest bidder for a set of better ones ?
They see something and make the assumption that it applies to all.
AND If they do know, they also know that a minimum of 95% of the people in this country are dumb-a**es anymore and don't know the good end from the dangerous end of a horse, let alone a gun and as such won't know any better and since they saw it on TV, believe it.