You got, basically, two types of slugs. You got your Foster slug. This is what people are talking about when they say "rifled slug". But the curved lead wings do not give the slug any spin. Instead they center the slug in the barrel, and then when it gets to the choke, which is smaller than the rest of the barrel, they squish down, and still allow the slug to be centered in the barrel. They fly straight the same reason a badminton bird does. Heavy nose and hollow back-end. These were designed for smoothbore barrels, and although you can use them in a rifled one, they work better in a smoothbore.
Then you got your "sabot slug". This is an hour-glass shaped hunk of lead that sits inside a plastic collar. The plastic grips the rifling and gives the slug spin, and then when it leaves the barrel the collar drops off and leaves the lead flying down range. They were designed for rifled barrels, and while they can be shot in smoothbores, the accuracy is not there.
There are a couple of other designs, but those are the two main ones.
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