I think a set of shotgun barrels is a great idea. For example taking a heavy rifle on safari. Switch the barrels to the shottie and enjoy a day's guinea fowl shooting or dove or whatever.

Also what a fantastic way to really learn your double rifle. Shooting endless clays with it. OK the shotgun barrels might feel a little different but it a start to knowing it like the back of your hand.

Also a set of barrels is sure lighter and less space consuming than a whole extra shotgun.