They can allegedly match the rifling marks left on bullets to barrels, but if you think about it, the barrels of thousands of guns are probably machined with the same tools, using the same drill press or whatever to make the barrels hollow and cut the rifling into them.
Also matching a caliber could be tricky since there are numerous cartridges that use an approximately 9mm sized bullet (among others). For example a 9x19 standard 9mm cartridge uses a .355" diameter bullet, and a .380 Auto bullet is .355" diameter, and a 38 Super bullet should be .356" diameter, and a 38 Special bullet should be .357" in diameter, and a .357 Magnum is .357" in diameter. But if those cartridges were loaded with solid lead bullets instead of copper encased bullets, they should be sized .001" larger in each case. And then they are being forced down the rifled barrels at extremely high speed and pressure and temperature. Lots of room for mixups
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.
They can allegedly match the rifling marks left on bullets to barrels, but if you think about it, the barrels of thousands of guns are probably machined with the same tools, using the same drill press or whatever to make the barrels hollow and cut the rifling into them.
Also matching a caliber could be tricky since there are numerous cartridges that use an approximately 9mm sized bullet (among others). For example a 9x19 standard 9mm cartridge uses a .355" diameter bullet, and a .380 Auto bullet is .355" diameter, and a 38 Super bullet should be .356" diameter, and a 38 Special bullet should be .357" in diameter, and a .357 Magnum is .357" in diameter. But if those cartridges were loaded with solid lead bullets instead of copper encased bullets, they should be sized .001" larger in each case. And then they are being forced down the rifled barrels at extremely high speed and pressure and temperature. Lots of room for mixups
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.