Easiest legal way to actually build one of these bad boys to fire is to make a SINGLE SHOT firearm. Order your 80% reciever without the mag slot milled in at all, and you can legally use the open bolt. AJ machine will do this for you. If you remove the butt stock and permanently alter your lower so you can never install it again, you can even keep the short barrel. (it will be a pistol, then) Otherwise, you have to register it as a short barreled rifle. Or install a 16 inch barrel and use the butt stock.