I'll try to find the thread but someone was replying directly to me as I recall and it sounded like they really knew what they were talking about. Perhaps even a lawyer. That is in regards to "possession".
ETA: http://legal-dictionary.thefreedicti...com/Possession
Read those. forget about "intent"
There are two definitions of possession "actual"(hold in hand) and "constructive"(your property locked in a safe)
The BATF probably does not clarify but you can rest assured they mean constructive possession. The simple test would be to answer this question. Do you own an SBR? The answer is yes, therefore you constructively possess a SBR. Placing a 16" upper on will not negate that fact. You would have to sell your short uppers to remove constructive possession.
Otherwise when the ATF answered that question above they would simply have said "No, stick a 16" on and go out of state". They clearly mean constructive possession not physical actual possession.
That is my opinion .This is NOT legal advice. It's simply my understanding.

