Every person in the world is a non criminal until they make the conscience choice to become a criminal. You are wrong in that there is this gray area.
So you want to decide which non-criminals should be allowed to get guns? Obviously you dont because its not possible. So how else would you like to limit how many guns are available to non criminals?
Are you actually trying to tell me that if guns became less plentiful it would make the illegal gun trade disappear? Do you not think that other criminals would not simply move in to fill that void? This is why I bring up prohibition.
Since more people are killed with cars every year, why not limit cars too? That would save more lives than any gun ban would. Also, you have a Constitutional right to your guns, not to a car.