Actually this isn't true at all. The fins on foster slugs do nothing to impart spin on the slug. They are there to crush as the slug goes through a constriction in a bird shot barrel or to make up for the difference in sizes between different manufacturers barrels. They stay stable in flight because they are hollow and have most of the weight forward, much like a shuttlecock in bad mitten.

You can shoot rifled slugs out of a rifled barrel, some are pretty accurate actually until the rifling fouls out, which could be as short as three or four shots. They even make some rifled slugs for rifled barrels using a special coating and/or harder alloys in the lead to reduce fouling.

A sabot bullet, especially a modern sabot bullet normally has better ballistics and performs better than a foster style slug though.

What you DON'T want to do is shoot sabots out of a smooth bore. Complete waste of money since they are designed to stabilize by spinning while moving through the air.

Paul