Thanks for the response. I was under the impression that ballistic fingerprinting was accurate. After you pointed it out, it is fairly easy to see why it isn't.
You are right about it being fairly difficult to enforce a lock-and-key law. However, it might just prevent some of the accidental shootings by children that we hear about. It might have also prevented a case like the shooting in Minnesota if the grandfather had had his revolver locked away. The real reason that I want to enact such a law is to hold irresponsible gun owners responsible if something does happen.
As for holding gun owners responsible for their guns falling into the wrong hands. If the gun owner kept their gun in a built in safe as I proposed, then someone robbing their house would not be able to steal their guns. It is more of an incentive to keep the guns safely stored.