Instead of "conspiracy to ...", the term you're looking for is "constructive possession" and the term comes from
US v. Kent. (Generously hosted by Quarterbore)
Hopefully, most people who rely on using a standard buffer tube are basing their reliance on informed decisions, having read both
Kent and
U.S. v. Thompson/Center Arms Co discussed in
Kent. (the sound you hear now is the sound of hundreds of ARFCOMers rolling their eyes, muttering "not this again...")
In
US v. T/C, the Supreme Court ruled that, under the NFA, if a configuration of parts, sold as a kit, has a legitimate and legal configuration in addition to an illegal (ie unregistered SBR) possible configuration, the registration tax under the NFA does not apply (with the assumption that the unregistered SBR is never "made" from the parts -- such a case would be a clear violation of the criminal section of the NFA)
The court in
Kent examined the SC decision in
US v. T/C and stated that if you do not have a legitimate use for complete and assembled parts that could be easily combined to form an unregistered SBR, a jury would be reasonable to determine that you constructively possessed an SBR. (They also examined
Owens that stands for the principle that if you DO make an unregistered SBR, it is no longer constructive, but ACTUAL possession of an illegal firearm)
Moral of the story: Although AR pistols were not discussed in the cases, above, an argument can be made that they constitute "legitimate" and therefore legal uses for parts that otherwise have the potential to be used to create unregistered SBR's a la
Kent. This would apply to the short-barreled uppers as equally to the carbine buffer tubes. BUT remember that the issue in
Kent was whether the jury was reasonable in its verdict that they didn't beleive the defendant when he said he had the SBR upper "just for parts."
Therefore, it is conceivable that you can have
a closet full of no * AR rifles
with A2 stocks (none of them NFA firearms) or over-16" uppers and an AR pistol with a standard CAR buffer tube, and have a jury find that the CAR adjustable stock piece you have for "parts" in the "same small apartment" with the AR pistol that could be "quickly and easily attached to" the AR pistol buffer tube "with a minimal amount of time or effort" would constructively amount to possession of an unregistered SBR in violation of the NFA. [
* edited for key point raised below]
This is even easier for a prosecutor if, while showing off to your buddies, you show them exactly what your pistol would look like as an SBR by popping on that spare CAR stock piece to the unmodified CAR tube on your (heretofore legal) AR pistol.
Likelyhood of prosecution? You decide. (Kent had multiple legal issues that gathered LEO attention)
Cost of your defense, IF prosecuted, even if you win completely? It would make a dozen Form 1's look cheap.
Cost if you lose? 5-10 Fed time, and NO firearm ownership ever again.
People buy insurance, but they don't want to spend the extra $5-10 on a dedicated AR pistol recoil tube, instead they inists on using the beat-up, used CAR tube they already have, and toss the stock piece in the corner. Think of that $5-10 as insurance against any and all suspicion that you might be able to pop that stock piece right back on when playing commando at 2 a.m. in your underwear.
If the insurance idea doesn't sound too stupid to you, you might consider a screw-type front pivot pin that requires tools and time to remove for your AR pistol upper -- it'd have been a lot harder for Kent's prosecutor to have testimony that he could have popped the short-barreled upper onto an unregistered rifle lower if they had to deal with Loc-tite in the process.
There is a distinction between "It is not illegal!" and "It is not illegal, and I'm completely above reproach! You don't have a case, so go away and stop bothering me."
Advice is worth what you pay for it...
Cheers, Otto