I don't feel like looking it up, but I think ATF has taken the position that if you own an AR-15 that will accept M16 select-fire components, just owning the gun and the machinegun parts at the same address is "possession of an unregistered machinegun" under the "constructive possession" theory. ESPECIALLY if some of those full-auto parts are ONLY used for automatic fire and have NO function or role to play in semi-auto shooting.