"How are you going to stop them? With another law (or even an existing one)"
What? THere is a law that covers that. It's the instant background check that identifies felons, crazies and other prohibited persons to FFLs. Otherwise myself and friends will watch our own business. So the legitimate market is closed to those that have forfieted their rights, or lost them as a tragic consequence of mental illness. Those folks will have to deal with those characters that are not my friends, the prohibited persons in the felon and enemies of the US catagory.
You think this prevents criminals from getting guns? Really?
But look at it another way. If something is a "right" they why do you have to ask permission to exercise it? The egregious Brady law forces up to ask the government's permission to exercise a right. Hence it is no longer a right, but a privilege. So with the passage of the instant background check, the right to purchase firearms went away to be replaced by the privilige to purchase firearms at the government's discretion.