It may be too simple, but here's an idea I'll throw out there: Stop selling guns. At least certain types of guns. And make it just a teensy bit bureaucratic to discourage those on a whim. "Hmm, what shall I do today? Hey! I think I'll go stock up on a few automatic rifles and a couple of thousand rounds of ammo! Never know when I'll feel the urge to do some target shooting." What's that? You say I have to wait? And you want a shit ton of paperwork filled out? Repression!