I am confused (a common occurrence when it comes to the intricacies of law) by constructive possession and the NFA, especially as it applies to the AR. As an example, it is legal for an individual to own a 14.5" upper on an AR carbine receiver if the carbine receiver itself is on an approved Form 1 or Form 4. The issue is that the characteristic (the short barrel on a rifle receiver) that makes the gun a "firearm" per the NFA is not the serialized carbine receiver; it is the barrel length of the upper being less than 16" and/or the OAL of the firearm being less than 26". Yet the registered lower receiver is the NFA item, not the non-serialized upper.
So now let's assume I have a registered SBR and a regular 16" barreled AR. Both are under my control. While I would have no reason to do so, swapping the uppers on the receivers is illegal and would be creating an unlicensed SBR. It is also very simple to do if both ARs are under my control. It is most definitely not legal. If there is no intent to do a swap, the NFA act has not been violated. That part I get (I think). The part that confuses me is where is the line. In other words, assume I want to build a registered SBR and am awaiting my Form 1 approval. I purchase from my dealer a 12.5" upper. If I were to bring that upper home before the Form 1 is approved, that is a violation even if I never try to assemble the upper onto an unregistered lower because both the upper and lower are in my possession and I have the ability, if not the intent, to assemble an SBR. Correct? If I leave the 12.5" upper at my FFL, I do not have possession and control of it, and there is no potential for an NFA violation. Correct?
So what happens if I have a regular AR carbine that is assembled and a 12.5" upper that is not on the registered lower because the registered lower is NOT at the premises, say for repair or refinishing? Is that an NFA violation because I have the ability, if not the intent, to swap the uppers? Once the Form 1 is approved, should the 12.5" upper never be in a different location than the registered lower?
I want to be very clear that I am not trying to evade the law in any way. I just want to be sure that I am not going to inadvertently violate it as I am strongly considering going the SBR route on a few things.