People watch too much TV.
Five minutes into any cop drama someone walks into the squad room with a folder in hand: "The slug recovered from the vic is from a gun registered to a John Smith." Then the star of the show says "Let's bring him in for questioning."
well that's because it was registered when you bought it.....duh
that why the empty casing are in the box.....to show you it worked or to register the barrel imprint???