I shot a 3.5 in #3 shot I have good luck with it on small ducks. on bigger ducks I shot a bb or bbb. always a 3.5 in.. ducks are tough. I have shot them and it will bounce off. It just depends on where you hit them. But, if you think you are hitting them and not knocking them down go to a bigger size or faster feet per second or even a heavy or tungston shot.
shotem in the head not shotem til there dead!