I don't think I'd shoot slugs out of a screw-in choke that is tighter than IMP cylinder. You may find the IMP Cylinder gives up 3" to 5" groups at 50 yards, with decent, full sized slugs. I've not tried screw-in "rifled chokes". I've never checked them out and cannot say if they have lands tighter than the actual bore and bore-sized grooves or not. Since shotguns vary so much nowadays in size, the dimension of the screw-in chokes will vary. For example, the bore of my 12 bore Mossberg M835 pump's smooth barrel is .740". The rifled barrel has a .724" groove diameter.

If you want to shoot the newer sabot shells with their undersized, jacketed bullets, you'll need some form of rifling. I would check out Ed Huble's extensive thread on using various bullets in shotshell wads. A very good one, found to shoot well in 12 bore wads, is the Lyman sabot. It is .69 cal. and is said to shoot quite accurately from a smoothbore. Your mossy should be able to drive that 500gr. at over 1,700fps with carefully loaded ammo. No factory slug I'm aware of, can match that.

The standard 'rifled' slugs (Foster- no the rifling does not spin them in flight) have not shown in magazine writer's testing to benefit from rifling to any extent. Since they are comprised of pure lead, the do not seem to do penetrate well on heavily constructed animals. They will work (have worked in the past) on Grizzlies at very close range, but that is not recommended except at spitting distance.