It’s because of the rifling. Most rifling in weapons are cut into the barrels by specialized milling tools. The rifling in Glocks are pressed into the metal by something called drop hammer forging. the barrel blanks are slid onto a forming mandrel and then the rifling is hammered into the blank. this leaves less microscopic toolmarks that gives milled rifling its “signature”. However ejectors and extractor claws probably would give the bullet casing enough of a signature to identify a particular weapon.
CC
Yes. That is my understanding.
As is usually the case, they found the man, that led them to the weapon. Not the other way around.