How insane is it that you could own a deadly machine like an AR-15 and not need to register it?

[BGBD]

172. What clues are you going to get if you don't have the weapon in hand?

Guns leave unique signatures on casings when fired that can be matched to a particular weapon....but you'd have to have a database of these marks to be useful, and even then probably would be so incomplete that it would be almost useless. Even if every new guns signature was placed in the database it would still be missing all of the hundreds of millions of guns in circulation and all of the illicit "ghost guns" that are mass produced and snuck into the country or ones printed with a 3d printer in somebodies basement.

Add to that the fact that you would still need to recover casings to make any match. So a killer could just be sure to pick up their casings or use a gun that doesn't eject them in the first place.

When you take all of that into account there aren't going to be many cases solved using a gun registry database and practically no crimes that it would prevent. That's a very small gain to be traded for a law that would require huge political capital and would be fought in courts for years, if it stood at all.