That's interesting to know. I didn't realize there were rifled shotgun barrels. I have a box of slugs I can't remember what brand off the top of my head I'm thinking federal or winchester. The box says 1oz rifled slugs. Now I'm curious if it the slug has some kind of rifling cast into it or it's for a rifled barrel.
As George P. said, there are two types of slugs; Saboted, which have either a basewad that is attached, (Brenneke/Lightfield type) or surrounds the slug and is made in (usually two) pieces that separate from the slug after exiting the barrel. (Hornady SST, Remington Accu-Tip, etc.) The other type of slug, Foster slugs, (often erroneously referred to as "Forster" or "Forester" slugs)
do have helical grooves molded into them, ostensibly to spin the slug, and were designed for smooth bore barrels. To the best of my knowledge and experience, they don't cause spin, or at least not enough to make them accurate.
As a rule, Sabots out of a rifled barrel are the most accurate type. Then would come Brenneke- type ones from a smoothbore barrel. Then Foster slugs through a smoothbore barrel. Shooting Foster or Brenneke- type slugs through a rifled barrel does not increase accuracy much over a smoothbore, and will foul the rifling quickly.
A word about Foster slugs- each individual shotgun tends to prefer one brand over the other for accuracy. It's best to buy a 5 rd. box of each, test them
on different days, (so you don't involve recoil reaction into the equation. Shooting more than 5 slugs can cause your accuracy to drop.) and buy the brand that shoots best from your gun.
