That's pretty much it. Some states require 1 or more spent cartridges from a new gun (perhaps used as well, IDK). Of course the casing is completely useless as wear begins to change the characteristics immediately. Also, if you intend doing bad things with a firearm, and live where the casings are collected, a firing pin and/or barrel is relatively cheap. As far as I know, this system has never resulted in an arrest, much less a conviction.