My understanding of the law is that if you make a "zip" gun but it clearly looks like a gun and its for your own personal use not manufactured with the intent to sell that it's totally legit. No need to tell the ATF or even put a serial number on it.
Basically the same laws apply to 80% lowers.
[Remington 870 12GA][
Stoeger Model 3500 12GA][
Savage 116 .30-06][
Savage Model 10 Bull Barrel .223][
Marlin 336SS .30-30][
Marlin 39A .22][Marlin Model 60 22LR][
Beretta PX4 Storm 9mm][
Ruger MkIII 22LR]
They who can give up essential liberty to obtain a little temporary safety, deserve neither liberty nor safety.Terry V Ohio Commentary