Use me as an example. I don't own any guns nor do I have a criminal record. Let's say I decide I want to go shoot someone. What legislation would you propose that would not allow me to purchase a gun that doesn't exist today that would prevent me from carrying out such a criminal act?
"not allow you to purchase a gun," probably none, just make it difficult for you obtain the weapon giving you time to reconsider "shooting someone"
Think of it this way, there exists three elements to every mass shooting: the shooter, the target, and the weapon. There is little we can do about identifying the next Stephen Paddock, and the targets are so numerous we can't defend them all, but we can act on the weapon that makes it all possible