Why are you shooting 3 1/2 inch shells when you can get the same speed and knocked down power with a 3 inch. The only difference is a few less BBs in a shell. If you compare the two with the same speed velocity give-and-take a few feet per second. You will actually get a better pattern with the 3 inch then the 3 1/2 inch. I shoot black cloud 3 inch with pattern Master choke for black cloud at 40 yards with 3 inch shells I get a perfect 30 inch circle, if I use this same shell and shoot with the 3 1/2 at 40 yards I get a sporadic pattern. The trade-off is a few less BBs, cheaper to buy and my shoulder doesn't hurt me at the end of the day. And I have kill geese stone dead out to 59 yards and that no B. S . With 3 inch shells. Try this experiment and let me know if I am correct or not.
Good luck
Ken