It may not be, but the second part of your sentence is why I usually err on the side of caution.
It's legal for me in my state to build a rifle from an 80% lower as long as it is for me personally.
The issue I see is IF you were ever questioned on it how do you "prove" that 80% lower wasn't built as a rifle first? You can't build a pistol from a lower that was previously a rifle, conversely they would need to prove it was a rifle first. I just don't want the headache.
When I take my pistol to the range, I carry a copy of the 4473 showing it marked 'other', I carry a copy of the pistol registration form as well as the BATFE letters regarding the 'brace', because I know if ever questioned there will always be that "one" cop who will argue, lol.
