Once a machine gun, always a machine gun. You cant go backwards. Same as if you fix a rpg, it becomes a destructive device again. Least that is how i always understood it
First off, remember the ATF has a tendency to change their minds about their interpretations, particularly in recent years.
Historically, and I believe still the case today, if an item is demilled/deactivated to or beyond ATF standards, it is no longer an NFA item. The receiver (serial number) should be removed from the books. No idea how good the ATF is at doing that, but it is what should happen, as that item is no longer a functional firearm.
The "once a machinegun, always a machine gun" rule applies to functional firearms. Specifically, taking a machine gun, removing the FA function, and leaving a functional semi auto. Under the ATF rules, these guns, while not functional full auto currently, they WERE at one time, and so the ATF does not allow their sale to the public.
For an RPG, the NFA part is the explosive warhead. IF you restore that to functionality, WITHOUT prior ATF approval, you're committing a crime.
OK, I now understand what is meant by "M16A1 receiver stub", but I don't understand the WHY one would want one....
If the OP returns, perhaps he could explain it??