Slugs through a cyl bore is fine. At my local range, we aren't permitted to shoot anything but slugs, and on my last trip (the day after Christmas) I shot a 5 round group at 25 yards with my Mossberg 590 that measured just over an inch. I was trying out the Brenneke/Kent KO Slugs. I was pretty happy with them. In any case, shooting them through a Cyl bore barrel is absolutely fine, and quite fun. Just don't waste your money on the really expensive stuf like the sabot's - they wont perform well at all unless you pick up a rifled barrel. The other stuff, like the brenneke's prefer a smooth (cyl) bore barrel.
-Spyderman