Yeah. Good point. There's not much way to keep somebody from doing something if they want to do it badly enough. Rescind the Second Amendment entirely and you'll have people making Sten guns and the like.
I'm not confident about the idea of total deregulation, but given the reality of the American context, maybe the only thing that would make a potential shooter think twice would be the knowledge that somebody would be shooting back within seconds of his/her first shot. But that brings about its own can of worms. Hell of a fix we're in.