Simmons ProDiamond 4x32 fixed scope. Simmons has a less than stellar reputation, BUT this particular scope, designed for shotguns, has received very good reviews, particularly from the muzzleloading crowd. It's rugged, clear, and easily good for shots out to 150 yards. It's only about $60-70.
Another vote for the Simmons ProDiamond scope. The ProDiamond scope on my CVA muzzleloader lasted through at least 1,500 shots. According to the present owner the scope is still clear and bright and on target. My rifled barrel model 870 shotgun also wears that scope.