I've got a friend that has his Class 2 Manufacturer's License. We were speaking about a related topic and he said that a machinegun is a machinegun. A machinegun is not a pistol, nor is it a rifle. Therefore, if you have a legal (registered) M-16 with a 20 inch barrel, and you decided to put a little 7.5" kitty-kat barreled upper on it, you aren't making a short barreled rifle, you are changing the barrel length on a machinegun.
I have heard others say that if you change to a shorter or longer barrel than was listed your Form 4, you simply change it back to the original length after use when you store it.
So - if you can put a barrel of a different length on it temporarily, why can't you temporarily remove the stock?
NFA is a whole different world.