The obvious solution is to tax ammo proportinal to its military use. You want military grade armor piercing rounds to shoot tin cans at your birthday party? Fine, that's taxed 1000%. Normal 9 mm rounds suitable for handguns should obviously be taxed lower. It doesn't ban firearms, it just creates a supply problem for nutcakes and preppers buiding arsenals. Security companies and firing ranges could recover their ammo tax at the end of the fiscal year. Hobbists could shoot assault rifles at firing ranges, as opposed to their back yard. The police won't need surplus military gear then. Your neighbour won't be wounded from a stray bullet shot from an assault rifle ten buildings away.

I think you guys have laws forbid civilian possession of military AP rounds in the US.