Good point. They could match the firing pin strike. Is that accurate enough for a conviction? I’m still curious about striation marks. It seems that shot, even through a rifled barrel, wouldn’t be able to be matched. Slugs yes.
Firing pin indentation, scratch marks from the load/unload mechanism, trace elements in the lubricants, any striations imprinted from the breach block or inside the chamber, residual plastic tracked in from the casing, trace elements in the brass alloy of the shell deposited in micropores of the chamber or and part the brass touched in its travels through the gun, skin cells common to any surface on the gun and the hull.
If they really want to find evidence they will. Even if it’s not really there. If you torture the data enough, it will confess...