You can take a factory (virgin) AR-15 Lower Receiver that has never been assembled and place a 7.5", 11.5", or 14.5" Upper Receiver on it. Make sure to use a Buffer Tube Rather Than A Stock. Also, you might look into Olympic Arms Pistol Buffers. They have a Buffer that doesn't stick out the back like most others do. Not sure whther or not it can be used on a Non OA Pistol Upper. When you are done place the Vertical Foregrip on the Gun and voila, you have an AOW. It can have a Flash Suppressor, Threaded Barrel, Grenade Launcher, Bayonet Lug, etc...or for that matter any non-nfa item. Unless, the gun is prohibited by 922(r) or the 1989 import ban. This would mostly apply to AK-47s, MP-5s, G-3s, FALs, etc...
If you use 10 or less than 10 foreign parts it is no longer regulated by 922(r). I know for a fact that MP-5s, AK-47s, and SKSes have enough available American Parts to be built into a legal config.