Try and shot shells that have the same speed as what you are used to when shooting traps.
Next when you try them on paper select the heaviest shot that gives you the densest pattern.Having the shots close enough so the head would get at least 2 pellets every time is good.
Then on the range at 30-40yards shoot at a 1/4-1/2 inch plywood. If it goes through this at that yardage it will kill ducks.