I too get a 3" group with a smoothbore Mossberg 500 at 100 yards. My set-up is the Mossberg (of course) with Winchester Super X rifled "foster" slugs and a old Tasco shotgun scope. I sighted the gun in at 50 yards with the scope and did alittle tweaking out to 75 yards - At 100 yards I have to hold about 1/2 - 1 inch high, but I do get 3" groups.

My cousin uses this set-up as well and killed 2 doe at 90 yards back in 2003 back to back - 2 slugs on 2 deer in the vitals - 90 + yards.

My gun shots the winchesters very well....