Question about SBR/Pistol length AR uppers
So I've seen on Gunbroker a lot of short barreled AR uppers for sale and they all say "all NFA rules apply". If I were to build an AR pistol with a pistol lower and buy the short upper (7', 10.5', etc), is that something that would need to go through the rigamaroll for NFA stuff since it's a pistol being built? As far as I know only the lower needs to be registered for an SBR, so if it's not being used to build an SBR does it need to be registered?
If I'm way off here please correct me. Thanks!