Rilfed slugs are really the only option for smooth bore barrels as there is nothing to stabilize the slug when fired through the barrel, so the slug itself must be able to self-stabilize. Sabot slugs require the help of rifling in the barrel to impart spin when fired, so accuracy of a sabot from a smooth bore is likely to be horrendous. An exception that I can think of is the
Brenneke KO sabot slug, which has grooves cut in the slug that supposedly impart spin even in a smooth barrel, so the box states it can be used in both types (rifled/smooth) of barrels. They're not cheap though. Any time the word "Sabot" appears on the box, expect at least $2/slug or more to be the going rate.
I've tried various brands of rifled slugs (Federal Tru-ball, Remington Sluggers, Winchesters, etc.) through my rifled Rem 870 barrel on a number of occasions. With some brands of rifled slugs (fired through a rifled barrel), I noticed occasional odd behavior, such as a slug going through the paper sideways, creating a rectangular hole in the target. In other cases, groupings from rifled slugs in my 18" smooth bore barrel were similar to that of my 24" rifled barrel. Overall, I consistently found the tightest groupings using sabot slugs in the 24" rifled barrel...in my particular setup (Rem. 870 w/24" rifled barrel), the Hornady SST's punched 1-1.5" clover leafs at 100 yds with no drop whatsoever. Best group I could get with rifled slugs (Rem. Sluggers) in my 870 (smooth barrel) was 2.5-3" at 100 yds, but with 3-4" of drop. This is good enough, however, for hunting deer, and definitely an inexpensive hunting setup.
Regardless, IMHO a rifled barrel *should*, in theory, give you better accuracy with just about any ammo fired through it, since it imparts spin on the slug giving more stability in flight. If you're just looking for slugs for plinking or for general shotgun handling practice, ammo quality isn't as much of a concern. However, for hunting, why not spend a little extra on a good quality sabot slug that gives a tight grouping (and much greater ft/lbs. energy) at 50-100 yds? I'd rather go with a consistent hitting, higher energy sabot for hunting purposes. For plinking or recreational target shooting, the cheaper rifled slugs are a logical choice.
If your goal is to hunt...you can do well sighting in and shooting cheap rifled ammo consistently with good groupings, then it's probably ok for hunting. Just make sure you stick with the same ammo that you use for sighting, preferably from the same lot/case, otherwise your POI may be off when you're ready to take that critical shot. Buy up several boxes of the same brand of slugs at once, sight in with 2-3 boxes, and leave things alone until you're ready to hunt.
