Actually you don't have to have a stock on a rifle. Look at folding stock rifles with their stocks folded that are still operational.
But the stock is still attached. A rifle with the stock removed no longer meets the definition of "rifle".
(7) The term "rifle" means a weapon
designed or redesigned, made or re-
made, and intended to be fired from the
shoulder and designed or redesigned
and made or remade to use the energy
of an explosive to fire only a single pro-
jectile through a rifled bore for each
single pull of the trigger.
Once the stock is gone it cannot be intended to be fired from the shoulder, so it's no longer a rifle. The folding stocks are just for ease of storage and transportation, still intended to be fired from the shoulder. Whether you actually do isn't the point, it's intended to be used that way, so it's a rifle.
A rifle with the shoulder stock removed would only meet the definition of "firearm".
any weapon (including a starter gun)
which will or is designed to or may
readily be converted to expel a projec-
tile by the action of an explosive; (B)
the frame or receiver of any such
weapon; (C) any firearm muffler or fire-
arm silencer; or (D) any destructive de-
vice. Such term does not include an
antique firearm.
We see the same with the pistol grip only shotguns. Teechnically they are not shotguns, since they are not intended to be fired from the shoulder. They are simply "firearms" and as such ATF has said that "firearms" must be sold under the same age restrictions etc as handguns, not rifles.
What I'm asking about is the miniature gatling gun shown in post 18 and Naolith's response implying that even with the barrels that appear to be shorter than 18 inches, it would not be subject to NFA rules because of it being a Gatling gun.
The Gatling gun refrenced above is also simply a "firearm", at least by definition. Same as the semi automatic Browning belt feds you see sold. Not rifles, since they have no stock and were not intended to be shoulder fired.
They would still be subject to the 16 (or is it 18) inch minimum barrel length requirement or they become NFA items "AOW". Maybe
I'm not clear on whether it is 16 or 18 inches. That one is not clearly addressed. The definition of SBR seems to imply that the rule only applies to rifles, which gatling guns are not. Fuzzy legal ground, and caution is a good idea.
The gatling gun manufacturer in the link above says:
Finished dimensions: Overall length 21 in.(gun) / 39 in.(gun and carriage). Overall height from floor 20 in. Scale field carriage has 18 in. diameter wheels. Breech diameter is 2 3/8 in. Uses 10 .22 cal. barrels, each 12 11/16 in. long by 0.465 inch OD.
So, going by SBR rules that breaks both of them, under 26 inch overall and barrels under 16. But, not being intended to be fired from a shoulder, is it still subject to those rules?
Probably end up one of those things where the ATF "interprets" the law however they want. Never forget that this agency made every shoestring in the US an illegal machinegun for a short time.
That's why it is so important to be careful when making your own, you can cross a line and be in big trouble quite easily. Most people end up writing tech branch of ATF to ask their interpretation before building anything.