For your bird barrel, you probably do want to usea rifled slug. Such slugs have been used in smoothbore birdguns for about 70 years with no problems. And if you should buy a rifled barrel or a rifled choke tube, you probably want to use a slug designed especially for rifled barrels. ...
These days most of the major shotshell manufacturers also offer sabot slug loads for 12 and 20 gauge shotguns. These are for useonlyin fully rifled barrels. How a long arm with a fully rifled barrel can be termed a "shotgun," I fail to understand, but that is beside the point. These...