RE: Accuracy with Smooth Bore and rifled slug
This kind of performance is not at all unusual for a smoothbore shooting Foster-type slugs. BUT, some other brand of slug might shoot a lot better! I have an Ithaca Deerslayer smoothbore that will shoot 3" groups at 100 yards, but only with one certain lot of Remington slugs, and no others!! I recommend you try different brands of reguar Fosters, and also some of the various "Brenneke" (those with attached wad columns)types. You will probably discover some type of slug that will shoot better than the ones you described.
Keep yore powder dry!!