Excellent point, Justin.
The salient point is that "Constructive Possession" is STILL left open by that ruling: (The full citation is in
ATF Ruling 2011-4 pages 2-3)
ATF 2011-4 said:
However, the Court also explained that an NFA firearm is made if aggregated parts are in close proximity such that they: (a) serve no useful purpose other than to make an NFA
firearm (e.g., a receiver, an attachable shoulder stock, and a short barrel); or (b) convert a
complete weapon into an NFA firearm (e.g., a pistol and attachable shoulder stock, or a
long-barreled rifle and attachable short barrel). Id. at 511-13.
So if you have a spare upper for your SBR, and say you have a receiver that is for your next ubercool-AR-flavor-of-the-month build, (but don't yet have the 16"+ upper for it) it could still be argued you are in constructive possession of an Unregistered SBR.
So the general rule still stands: Have enough parts to make every lower you own a "legal firearm." Then have all the spare uppers you want, provided you can make legal firearms with those uppers.
However, if you build that receiver into a PISTOL FIRST, then you will have no issues, since per ATF Ruling 2011-4, you are perfectly LEGAL to build a PISTOL into a RIFLE and into a PISTOL AGAIN without violating NFA rules.
Clear as mud?
Now, how do you PROVE you made your spare ubercool-AR-flavor-of-the-month lower into a pistol FIRST? I don't know. I would take a picture of the pistol build on top of the newspaper on the day you build it. Then you have SOME proof of time line. Much more than the "man" could do if he decided to try to pinch you for it.
As for the Thompson Center Case, remember that the TC Encore was ONE receiver with multiple parts. It didn't come with "multiple lowers."
Please understand, I am not a lawyer, but I have these discussions with them on a frequent basis. This is OFTEN the topic of discussion.