I can envision the drug cartels branching out into guns and ammo. They supply where there is a demand.
Something similar has occurred in the U.K., where sub-machineguns from the former Eastern Bloc countries have crept in, replacing the banned handguns.
Since the penalty for possessing handguns is the same as that for the fully-automatic weapons, the "In for a penny, in for a pound" reaction was totally predictable.