Perhaps they would. But while cost is a factor, it doesn't seem to be the main factor in the selling price of anything. Chances are, were they suddenly legal, just supposing, it would be quite a while before the price went down and they might even go up first.
On the other hand, maybe I'm having trouble thinking of an M-16 as a machine gun.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.