2 oz. of #4's work for me. Not much field data except my first two birds last spring.
First one, good head shot, bird fell over and never quivered. Second one, blew the shot and quit swinging as he was walking out. Missed the head but broke the left wing and leg (fortunately one of each so he couldn't get away.)

IMO inside 40 yards, a good hit should do the job with either size shot. Berreta, 49 yards? Come on, Man. I believe you, just question the judgement.

Romans 8:28