The most accurate rifled slug in my mossberg 500 is a 2-3/4" Federal. I get 6" groups @ 100 yards with an I.C. choke tube. Plenty accurate for hunting. This little buck bit the big one at 75 yards, hit him through both lungs and he went around 60 after the shot leaving one heck of a blood trail.
I now have a Remington 870 wingmaster that shoots several brands of 2-3/4" rifled slugs through an open choke uncommonly well. I get 3" groups @ 100 yards with that one. Never had a chance to get a deer with that set up though.
The Mossbergwill not shoot 3" slugs worth a darn. Then I have a 20ga H&R that won't shoot any slugs well enough to consistantly hit a paper plate at 25 yards. It all depends on the gun.