Something else that I have not seen brought up here, but is very important to the discussion:
INTENT.
The ATF has established, multiple times over, that
intent to use something in a particular manner is enough to create an illegal NFA item. ('Constructive possession' is based largely upon this premise, as well. -Even though the intent might be a fictional construct of the ATF/FBI/etc.)
If you build a "pistol" and add an 'arm brace' with the intent to shoulder the 'arm brace' and use the pistol as an SBR, you are committing a felony act.
Just because, "the ATF isn't going after the YouTube guys," doesn't mean YOU (and they) aren't committing an illegal act by building a "pistol" with an 'arm brace' with the intent to build a 'sleeper SBR' and subvert the NFA.
A case, which I recently dug up for another discussion, that was later used as a precedent for others:
United States vs Webb (1995)
(The above link is to a 1996 appeal.)
Webb was convicted of possession of unregistered firearms and firearms not identified by a serial number, based almost entirely on his
intent to break the law by constructing a 'silencer' - not the actual material construction or viability of the devices.
Why is
intent so important here? Because the devices in question were made of materials that the ATF admitted would not have been effective or survived actual use.
An except from the case files:
(...) the defendant made two suppressors from “[o]ld toilet paper tubes and stuffing from some old stuffed animals.”
Webb's sentence was reduced, specifically because of the materials used. But he
was convicted.
Note that the case also involved a, "Raven Arms .25 caliber semiautomatic pistol that had no serial number," but he was never even charged for that one. The ATF and Kansas Bureau of Investigation stood on the charges relating to the
intent to use cardboard tube 'silencers' and got the conviction there.
