So, by the ATF’s reasoning, if you own an AR pistol and an AR rifle of the same caliber, you could be guilty of “constructive possession”? You could, after all, swap a sort barreled pistol upper onto a rifle lower with a stock.
It's the same problem with T/C Contender and T/C Encore. If you own rifle and pistol barrels, you always have the ability to assemble an SBR from a lower with a rifle stock and a pistol barrel.
No. If you won a pistol lower with a buffer tube that won't accept a buttstock, you have a pistol. If you own a pistol with a carbine or rifle buffer tube, and you own a brace or buttstock that isn't already attached to another rifle, yes you have constructive possession of an SBR.
As long as you have a rifle upper to go with the rifle lower, you are not in constructive possession just because you 'could' put the pistol upper on the rifle lower.
If, however, all you owned were pistol uppers and rifle lowers, then yes. It's not supposed to make sense...