I also used to carry trap loads or heavier handloads using small shot for killing crips on the water. #6 in big gauges to #8's in the smaller 20 bore, using as big a shot load as I could get worked on the heads well, even at longer ranges. Due to the stationary target, the entire shot string would pass the bird's head, making long shot strings useful. I suspect a #5 shot or even #6 in steel would work as well as the 7 1/2's and 8's did in lead.
It is about impossible to penetrate a cripped duck's or goose's wing nor penetrte the body when sitting on the water. The head and neck become the only viable target then - small shot answers well for this, whether they are flying or 'sitting' wounded.
It did take some practise to use the head as an aiming point, rather than the huge body of the goose or mallard, but with a bit of practise, the system worked perfectly. I found nothing better for ducks over decoys, than # 7 1/2 trap loads in the 12 bore, or skeet loads for the 20 - #9's - riddling the head and neck with shot produced dead ducks - and geese, rather the wing broken crips. Did you know a broken wing'd goose can paddle faster than you can in a canoe? Been there - with Taylor.