Your suggestion would amount to infringement on the SA.  Also, there are literally billions of rounds of real ammo out there already -- gonna round it all up with giant magnets or what?  You reckon criminals would comply and only purchase rubber bullets, because there WILL always be ways to buy real ammo from the underground were your plan to take effect.  Also, how's that gonna stop someone from renting a U-Haul and mowing people down along a sidewalk?  Had enough "rational rebuttal" yet?