The ATF at one time declared the auto sear, (all by itself, no other parts or gun needed) to be legally, a machine gun.
Is that still the case? If so, how is it you can buy an auto sear at brownells with no paperwork at all? I know they're intended as replacement parts for existing NFA M16s but then it would count as a part not the actual MG.
As far as I understood the lower itself is the machinegun. Having all full auto parts in your AR15 is legal as long as it does not have a lower with the necessary cuts and pin holes for the auto sear. But the sear itself is not the machinegun, just the cuts and pin hole for it are essentially.