Also, by the letter of the law, you could get dinged if you're in possession of the SBR upper and don't have a legal lower for it. Meaning a pistol lower or a stamped sbr lower. You start getting into "constructive possession" and such. The easy fix for that is to have a lower sitting around with a pistol tube on it.
But this would be a ridiculously rare scenario. To be caught and charged this way.