Is that what is called "constructive possession?"
SBR "Constructive possession" with AR pattern guns is a bit more grey than it is with others:
With machine guns, having all the FA parts and a receiver that can accept them does constitute constructive possession, whether you ever assembled them or not.
With SBRs, any rifle can be made such with a mere hacksaw, but possession of a metal cutting implement and a title I rifle certainly does not constitute "Constructive intent" or "Constructive possession".
With AR's, however, the ease with which the short upper can be put on a lower means that a zealous agent
could try to prosecute someone who has both. That said, having AR pistols and AR rifles at the same time is perfectly legal and, though one could certainly put their pistol upper on their unregistered lower, to prosecute, the burden of proof for prosecution that the accused did so would have to be met. Basically, they would need a verifiable photograph or admission of guilt.
If one had a only a short upper and an unregistered rifle lower, it would ultimately be up to a judge whether the individual intended to assemble an SBR or if you planned to build both a pistol and a rifle, but hadn't acquired all the parts for either yet. Personally, if I had only a rifle lower, I would think twice about having a short upper around the house.
Me? I have 3 rifles, one pistol and a complete lower that's going to be another rifle. My pistol was built on a virgin lower, and as we speak, the form 1 is en route to ATF to SBR it. I do already have the stock I plan to use, but I don't have a buffer tube laying around that will accept it (well, it
could slide onto the pistol tube if I took the foam pad off, and if I didn't mind it being all the way forward and free to rotate 360* or just fall off)