UPHiker,
Would that also be true if the lower shipped from the factory with a rifle buttstock?
Probably not. There could potentially be legal arguments made if the first time it was fully assembled it was as a pistol, but I wouldn't want to pay what it would cost to make that argument in court.
If it shipped with a rifle buttstock attached, it's best to treat it as a rifle. IMO pinned and welded muzzle devices are a pain in the neck, so I'd stick to a 16" or longer barrel.