I agree with 2 3/4" slugs. There have been vast improvements in slugs and slug guns in recent years. They shoot flatter, are more aerodynamic, and much more accurate than anything my dad ever had growing up. Since your barrel is fully rifled, don't forget sabots. the Winchester loads shoot great out of my gun and kill deer dead. Nope, no 3 or 3.5s for me. Besides, trying to find the right load out of those shells will loosen you molars!