Modified is going to be about the best for accuracy but accuracy is not a real factor with pumkin balls in a smooth bore. You groups will be paper plate sixe to about 60 yards in most smooth bore guns. Some smooth bore guns like my father A5 with slug barrel will do 3 inches at 60 yards but my 870 with smooth bore barrel would only get me 4 inch at 50 yards. breneke makes so good foster slugs but the winchester pumkin balls are the cheapest and work about as well.
A cheap fully rifled barrel made by mossburg and sabots will get those 3 inch groups at 135 yards and If you find the prefect load much farther. it cost you about 2 dollars a shot for the sabot slugs and about 4 weeks of a sore shoulder when you sight it in.
I have an extra fully rifled mossburg barrel for the wingmaster if you might be intrested in it. Works great with federal barnes expanders and has the canntilever for mounting a scope. PM me if that is something you might want to try out for this season. I might let it go to a good home since my marlin and I have bonded rather well.
There are tricks that help many smoothbores shoot better.
1. Buy all your slugs from one lot# (assuming you have found a slug brand/type it likes)
2. Bbl fit
A. Bbl to receiver fit.......might need to put a layer or two of masking tape on bbl
flange and seat bbl with wooden block tapping at muzzle.
B. Mag cap snug, stay snug, but not too tight. May need equispaced.
3. Trigger job. If a dedicated deer or turkey rig, do it.
4. Scope setups should be low so the stock can be of proper cheekweld. Most cantilever
bbls are way too high for Joe Average.
5. Zeroing. Lead sleds and vises can help keep a gun from thumping you, but do not
replicate field positions. Learn to shoot it correctly without such odd tooling. Bags on
risers for fairly upright body position will mean your aiming points developed on the
range will transfer to same location hits on deer.
The slugs are kinda slow, the recoil a bit big, so gun movement during slug's bore
travel has to be repeatable. This is why slug bbls tend to be short.....less time of
flight = less time to absorb variations in form.
Worst rig I had was a Rem 870 Police smoothbore. Bbl fit at receiver was bad. I taped it up and got it shooting pretty good at 75 yards. Of course the deer walked by at 25. Mossberg smoothbores, 100 yards were easy, the three I had would do well to 150, think the bbl lockup at the front possibly why they shot so well. Built up 2 rifled 870's and used sabot slugs in those...........best slug rigs I've shot yet, 1.5-2" groups at 100 yds and they fit like a good rifle.
I've taken my 870 out to 225 yards, groups within the center ring on Dirty Bird sight-in targets, however big that is, good enough for me. It wears a Shurshot stock, cantilever rifled barrel, and a Nikon BDC Slughunter scope. No modifications. I'm happy with it. I hunt in IL(southern), so it's my only non-muzzleloader longarm option. Being able to get out to 225 yards is probably pointless because I rarely have a sightline open longer than 75. lol
Edit: I use Hornady SSTs, that's probably important