It's too late.
You can't cancel millions of weapons even if you do render them illegal, which no government, either state or federal appears to have the will to do.
According to France 24 tv just now, in Nevada - or at least in LV - you can carry a concealed weapon in the street; have no need to have a licence and the only thing that was attempted to make the issue a tiny bit less dangerous was to create a short period between buying a weapon and being allowed to take possession. This was to allow time for a character check. Turns out that the local govt. expected the FBI to do the checks and the FBI said no way, you do it. So nobody is going to do it. So no regulation. Bang, bang on!
The only way it may happen is if tourists start to desert Las Vegas in droves, and powerful hands get forced into action. Empty hotels and casinos will soon take care of a little legislative business, methinks. After all, it's money keeps the laws off the books, so more money can also do other things in the opposite direction.