Thompson-Center successfully argued differently to SCOTUS against the ATF. The ATF's current interpretation seems relatively straightforward. I'm sure a lawyer could elucidate further, otherwise or in some non-euclidean direction. But I've only ever heard them say the same thing the ATF's 2011-4 ruling says to my non-lawyer brain.
If it starts as a pistol (or at least, not a rifle) it can be converted to a non-NFA rifle and back again at will. Converting it to a non-NFA 16+" barreled rifle is not permanent. It's also why unless there's some peculiar state law (like CA, IIRC) if you start with a bare receivered anything always build it out as a pistol first so you have the flexibility to convert it to whatever non-NFA configuration you want when you want.
a firearm, as defined by 26 U.S.C. 5845(a)(3) and (a)(4), is not made
when a pistol is attached to a part or parts designed to convert the pistol into a rifle with a
barrel of 16 inches or more in length, and the parts are later unassembled in a configuration
not regulated under the NFA (e.g., as a pistol)