“In THIS case, they are in a tight spot, as projectile evidence from polygonally-rifled barrels is not reliable. I’d say they are far more interested in cartridge casings, as Glock ones are VERY specific to the individual firearm. Glocks have a characteristic impression that is particularly easy to match.”
Rectangular firing pins.
“Rectangular firing pins.”
Partly, plus the firing pin orifice on Glocks leaves a mark that can be individualized to the gun. Then there are extractor and ejector marks as well. All of these have been admitted as evidence after their Daubert hearings.