Either drop the dough on a rifled barrel, a rifled choke tube, or work through a boatload of loads that pattern decent. Are you sure your peep sights aren't F'd up? Start at 20 to 25 yards and work out. Plain Jane Winchester rifled sluggers arepretty accurate.

Also a really good write up in last months Outdoor Life about the best 12 and 20 ga slugs. Lightfields took the prize..