OU and SXS shotguns will always be 2 part, but those parts are not interchangeable, so this is going to be an interesting ruling.Well, to follow the law as it's written (I know, I know), neither the upper OR lower qualifies as a receiver, or a firearm. Which means, neither part needs to be serialized at all.
The law was written many decades ago, before the AR type weapons were even thought of. Nobody had conceived of a receiver that didn't contain the trigger group, and wasn't threaded for a barrel. Nobody had conceived the 2-piece design.
The way I see this going, is the 2-part design being negated. .Gov will force the 2 pieces to be irrevocably united in some manner. This possibly will spell demise for the modular rifle.