He'll use executive fiat, executive order, or he'll find some government agency to do his bidding much like he used the EPA to impose environmental mandates that he couldn't get through Congress.
If that doesn't work, it's been proven that judicial tyranny is another avenue for getting around the constitution for this dictator.....I mean President.
He could use the EPA again by banning the interstate transportation of toxic substances (lead).
Yeah, and I'll bet I know how: He'll force the USPS, and pressure UPS and FedEx to not ship the ammo. Buy it online all you want, but you can't get it shipped. That's the Chicago Way.