You can shoot lead slugs in a rifled barrel but it will foul badly. Sabot slugs have several advantages: less fouling, better ballistic coefficient due to the smaller diameter, better shaped projectile, and in some cases less recoil. Even so, not all slug loads shoot well in all barrels. I doubt that you should expect better than minute of deer accuracy at 100 from most pump guns with randomly selected ammo, rifled barrel or not. IIRC, claims of 200 yard slug accuracy came from quality bolt actions from Browning, Savage, and Tarhunt.