BGOS. An upper in any state is not a firearm. An incomplete 80% lower is not one either. I bought a PSA rifle kit with standard barrel and swapped the barrel for a $75 HBAR before mating it to a completed lower. Nothing illegal about any point in that sequence.
Not worth the risk. If you don't own a pre-2013 lower. Don't buy a skinny barrel upper and have it with your post Fsa2013 lower.