Cool story, bro. Now do bumpstocks.
Ok. No problem. Part of it requires careful reading of the law, because the ATF didn't rewrite it.
The term “machinegun” means any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger. The term shall also include the frame or receiver of any such weapon, any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun, and any combination of parts from which a machinegun can be assembled if such parts are in the possession or under the control of a person.
I bolded a section. The bump stock question is whether the parts exist and were intended to make the firearm shoot twice with one "function of the trigger". If they did, then the part (the bump stock) is a machine gun by itself. The initial approval letter hinged on the bump stock forcing your finger to actuate the original trigger of the firearm once per shot (twice actually, see binary triggers but that doesn't matter). The ATF has long held that devices that attach to firearms and actuate the trigger themselves take over as the trigger. You can see this in the infamous shoe-lace machine gun, and the electrical solenoid w/ button trigger (like a vulcan cannon, or paintball gun trigger packs) Yes the original trigger moves once per shot, but by automating the pulling you've "redesigned" the trigger mechanism so you only do one thing (pull the shoelace and/or push an electrical switch) and so one function of the "new" trigger shoots more than once, and those are machine guns. That is within the ATF's regulatory function (sorta).
On the bump stock, what the ATF actually did (as opposed to what Guntubers pretend they did) was re evaluate the part based on orders from the Executive and find that the bump stock, when installed, moved the "trigger" from the original lever to the pistol grip/handguard interface. So that one function (holding the handguard away from the pistol grip) fired several shots. That makes the determination that a Bump Stock is a Machine gun in and of itself, fall squarely within the NFA, as written. Because that section of the law is written to include a wide variety of posibilities.
In contrast the CFR on frames is much more specific:
Firearm frame or receiver. That part of a firearm which provides housing for the hammer, bolt or breechblock, and firing mechanism, and which is usually threaded at its forward portion to receive the barrel.
You'll note it doesn't leave any room for marrying parts together to meet the definition. It says Hammer, bolt or breechblock
and firing mechanism. They know it too.
Read this letter classifying the upper of a USC as the frame. ATF says "The upper assembly of the FNC rifle is more properly classified as the receiver" because they had to pick something. They say in their letter that the hammer is in the lower receiver, and included in the definition, but more receivery stuff is in the upper so let's go with that. And they can do that until a court tells them no. But once a court says no, it'll need to go back to a legislative body to rewrite that law and swap the "and" for an "or". They could try and do it with a letter ('ala bump stock) but they'd be in the same situation they are now, making rules and hoping no judge ever has to look too closely.
