I only shoot slugs a few times a year on a small private piece to get a backyard buck that fills the freezer. Living in CA, finding rifled slugs in copper is hard to impossible, but I found sabot slugs. I shoot em through a benelli super black eagle with a carlson rifled slug choke. That choke puts just enough spin on the hornady copper slugs to make them accurate. Longest shot has been 75 yards so far with just a bead sight. The copper doesnt expand well though imo at shotgun speeds and tends to just blow a small hole through them. Its enough to kill though.
So keep that in mind for your current smooth bore gun - carlson rifled choke tube. Cheaper and easier than a new barrel or gun, will get the job done.