re: Updated: Full auto lower laws.. Answer from ATF on page 2

So before I started this thread I emailed the ATF not thinking that they would actually email me back with an answer. Much less with an answer the next day. Here is the question and answer.
Questions:
Scenario 1:
Say I obtain a tax stamp for a machine gun in the form of an ar15 lower and when bought it comes with an upper that has a barrel less than 16 inches. Does the tax stamp for the machine gun lower cover the fact that this would also be considered an sbr due to the fact that it has a barrel less than 16 inches ? Also can I then take the machine gun lower and swap it around to a number of different uppers without any legality issues and if so is it ok that I would then have an upper laying in my safe that is not attached to any lower but it is still an upper with a barrel less than 16inches with no lower registered as an sbr lower to host it.

Scenario 2:
This scenario is fairly interconnected with the first but say I am in the process of waiting for my tax stamp to be approved for an sbr. Is it completely legal to have all components for the sbr including a barrel which is less than 16 inches and the lower that I plan to be serialized as the sbr lower as long as none of the pieces are assembled to become an unregistered sbr.

Answer from ATF:
Generally, a weapon registered as a machine gun would not need to be re-registered as a short barreled rifle if it had a barrel less than 16". If you change the barrel length, overall length or caliber of your registered machine gun, notify us in writing and we will update the registration database.

A firearm, as defined by the National Firearms Act (NFA), 26 U.S.C. 5845(a)(3), is made when unassembled parts are placed in close proximity in such a way that they: (a) serve no useful purpose other than to make a rifle having a barrel or barrels of less than 16 inches in length; or (b) convert a complete weapon into such an NFA firearm.

So there yah go. Take that for it it is.