Besides, since barrels leave unique marks on the bullet, we’ve got traceability there. And I know some of my new guns come with fired bullet so I figure someone is likely collecting that bullet mark in a database somewhere.
The uniqueness of the marks from the barrel has been shown to be far more dubious than presented on TV, and in many court cases. It’s more like a shoe-print, in that it is still useful, but not unique.
Further, those marks change over time.