Very interesting question. The basic reference line in shotgun design is the barrel axis, and the barrels are the firt bit that is put together in gunmaking.
However, the barrels are the most "consumable" bit of a gun, and while you can barrel an action you cannot action a set of barrels, unless you want to spend more than the price of a new gun.
So yes, when buying a gun for a rebuild you buy the action. And in this context you can appreciate the technical merits of monobloc barrels since in the case of a monobloc equipped double you retube rather than rebarrel, a process much less costly than rebarreling or sleeving.
