One thought is to try 3" shells first and then decide on the 3.5" shells. Be sure of the POA/POI from top and bottom barrel regardless. It can be critical at 30 vs. 40 yards,but usually not a miss necessarily. I have an O/U 3" 12 and a SXS 20 gauge and recoil difference is huge. and extra weight is noticeable in the 12 ga.