You can prevent the gun show loophole (which I am for preventing) without banning everyone from owning firearms.
I've answered this before, you just hear what you want to hear. If you could get everyone to turn them in, and you were able to ban them all, of course it would be less likely. But you still haven't said how you would ban them. The functionality of this ban isn't possible - that's your problem. You can't just declare to the land that these guns are illegal. That doesn't do anything. You have to detail how you would get a list (there is no such list) of all owners of said weapons. Then how you would get them. Simply making it a crime to own such a weapon suddenly makes millions of people criminals. No one is going to stand for such lunacy except a few crazies like yourself. You're the fringe of this movement. This is what you don't get.
There are plenty of Americans that are for common sense gun laws that don't currently exist, or enforcing the ones that do exist. You don't bother with any of that. You just put the peddle to the floor and demand all semi auto guns banned NOW! and you scream this during every argument. No methods, no logic, just emotion. That's your problem. It's always been your problem around here.