The machine tools used to cut the barrels and other parts have replaceable cutting elements, but your point still stands - nobody is keeping track of how many barrels or which barrels were turned with what cutter on what day.

There's an even more damning argument, however. All a potential murderer has to do is shoot a couple of boxes of rounds through their gun before shooting their victim. Then they clean the gun and would you look at that, the clean barrel makes different marks than the dirty one.

Edit: I remember that I could fire .38 special through my .357, and I often did for target shooting because it was cheaper.