I have researched this and have yet to see a definitive answer with corroborating documentation. Lets saw you have a stripped lower that is NOT registered as a pistol (or SBR for that matter). Build out the lower with a pistol buffer tube and NO stock. Can you place a shorty upper on the lower as built without risking a trip to the gray bar hotel. Alot of misinformation out there. Wanting to get a straight answer. Thanks in advance.