Rifled slugs (foster) have rifling cut into the slug. The are designed for smooth bores. Those are what you saw for two bucks. For a rifled bbl you need sabot slugs. The slug is encased in the plastic sabot down the bbl and then is discarded after leaving it. They are the ones that were expensive. But that is what you need for accurate shots out to 100 yds out of a rifled bbl. Hope this helps.