It requires a goodly amount of firearms savvy to modify a gun from semiauto to full auto.
Gang-bangers, who commit the lion's share of firearms crime, couldn't modify one in such a manner if their lives depended on it.
Most law-abiding "gun nuts" , like myself, have never modified one nor even attempted to modify one. In my half-a-century on this orb I've never seen one modified to full-auto, and I have seen a LOT of guns.
Thus, most laws to control crime committed by semi-autos modified to fire full-auto are exercises in futility. Most studies I have seen on this subject estimate that less than 1% of all firearms crimes are committed by such weapons.