All a rifled slug 20 or 12 gauge is, is a big bore rifle with a plastic straight wall. then why must I use a sabot???
Not quite. Cast-friendly rifles have a proper throat. Shotguns may have an oversize forcing cone that the slug must leap across.
Cast-friendly rifles have proper twist. Dunno about slug guns. I have heard the twist may be a little fast.
But some people are shooting full diameter lead slugs in their shotguns. I don't have experience in that area so I can't give any detailed advice.