My gun is a 2 3/4 chamber... we were using #3 (Federal as I recall). My Golden Eye was about 25-30yds when I popped it, it rolled belly up and splashed in the pond... Christmas was cancelled. My buddy was surprised, given how tough those little fellers are. But, then... the head shot definately helped to seal that deal. My little teal was about the same distance, and had JUST tucked it's wings after landing infront of me. A wring of the neck finished the last of the twitching he had left in him.
My buddy was shooting a Baretta auto-loader (same loads). His drake Mallard was dead before he splashed down ('bout 30yds), the hen splashed, but righted and was trying to escape the dog and needed to be finished with a neck wringing. They were paired as they were cupped and dropping in, he tagged her first. The Mergansers needed a neck wringing to finish 'em off too.