This oughta be made a huge typeface sticky here.
Rifling on those slugs is there purely to allow the outer surfaces of the slug to swage down in passing through smoothbore chokes. It is not there to make the slug spin. It is neither restricted to smoothbores only nor prohibited from rifled barrels (two more common misconceptions).
Rifled bores were not built or developed exclusively for saboted bullets (usually called sabots), they were used with standard Foster type rifled slugs for many years before sabots were developed.
Smoothbores were also used with rifled slugs for many years & I'd consider it safe to say there were & are many, many, many more rifled slugs put through smoothbore barrels than rifled barrels.
Denis