Seems to me if it doesn't fall within the definitions as specifically given for NFA, it becomes a "firearm". For example, if I throw my SPR upper on my pistol lower. It's not a "pistol" or "SBR" length upper, so it's either a firearm, or simply a pistol, regardless.

If the PGO doesn't meet the onus of AOW or SBS, it's simply a firearm.