Mr.L wrote:, a mallard drake cupped in and hovered over the decoys at about 30 yds. I shot him with a Black Cloud 3 in. #4 shot. He folded up and fell in the edge of the decoys. We had a group of three mallards come in. I raised up and shot. My first shot just knocked feathers. Lastly- I fired one shot and he helicoptered dead.
I'm sure you know some of this but the way the bird reacts will often tell you where you hit. Frist bird that crumpled hard but swam off was likely a high wing break where the main wing bone is broken just outside of the body causing them to crumple hard but once on the water they make a beeline out of there. This is pretty common on birds that are wiffling into the decoys and moving side to side causing a shot to be off to one side some.
Second bird that puffed feathers was likely hit towards the rear of the body where feathers come out more easily. It gives the appearance that you slammed the bird but there's nothing vital in the back half of the body no matter what you shoot them with.
Third bird was head shot. Every bird that helicopters down will have a pellet through the cranium.
I prefer the 2s over 4s but where your pattern was centered or just luck of where a pellet struck likely had more to do with those three birds than the shot size.