Your claim that a bullet can’t be proven to come from a specific gun without it being witnessed being shot is incorrect .
Weapons recovered from crime scenes are test fired, the bullets recovered then compared under a microscope along with bullets/cartrige cases recovered from the crime scene & the deceased.
Using microscopic comparison of rifling marks on the bullets as well a marks put onto the cartrige cases by the bolt face,extractor ,ejector,magazine feed lips,chamber of barrel bullets can in fact be matched to specific weapons .
Probably with about the same alleged certainty as fingerprints. :-(
In the end a determination is made by fallible humans.
My original point was responding to the claim that a bullet can be identified as coming from a specific type or model of gun, without having the gun to compare it to.