RE: proper choke for rifled slug
With any slug the more open choke you have the better your accuracy will be. Tighter chokes deform the slug and effect accuracy for the worst. Improved cyl is what I use.
You can shoot rifled slugs out of rifled barrel or rifled choke, but they aren' t made for that. Some guns will give you better accuracy, I used to use rifled slugs in a rifled barrel for practice because they are cheap. But you have to clean it like you would a muzzleloader, it' s very dirty and you won' t get much better accuracy than with a smoothbore. Rifled barrels and rifled chokes are made to shoot sabot slugs.
All brands of rifled slugs will shoot different in each gun. Find which brand shoots best in your smoothbore. Some guns will pattern very accuratly but will hit where the sights aren' t lookin, if that is the case, you need to either get rifled sights or a scope. Or just use kentucky windage.