Why go to 28ga in the twelve? He can either reload as JDW suggests (best option in my opinion) or use the chamber inserts for 20ga which will perform better in the 12ga barrel than the 28. The skirt of the 20 wads seem to stretch enough to provide a better seal than does the 28ga. Neither 20 nor 28 gauge in the chamber inserts will be useful beyond skeet ranges in my experience.