Is there a reference for this above info?
I did find this rule clarification from the ATF: http://www.atf.gov/files/regulations-rulings/rulings/atf-rulings/atf-ruling-2011-4.pdf
It basically states, that receivers can be 'assembled' as a pistol, and to a 'rifle' and back to a pistol again as long as the laws are followed about 922(r) compliance and NFA laws.
I still have not seen a ruling or holding that JUST BECAUSE a weapon receiver (the serial numbered "WEAPON" part that is the WEAPON, & not just parts) CAN be made into a pistol, it HAS to be transferred as a PISTOL.
If someone knows WHERE this is, please reference it.
Another belief: A firearm that has started life as a rifle, CAN NOT be made into a pistol, due to the wording in the law. You can go from pistol to rifle, and now back to pistol, but you CAN NOT go from rifle to pistol. ?
Confusing as hell !
EDITED TO ADD: Thanks MedicMan & Tactical Coyote! I sincerely appreciate the references!