I agree with the above and would add that it's very easy to misread where your pattern went, by what you see as impact on the ground, in tree leaves, or on the water. I'm not saying you are wrong about having the bird in the center of your pattern, but at least accept that it is possible that you misread it, and that you didn't. Have you patterned the gun with these loads? And, maybe more importantly, have you shot the gun on a plate or large paper to "register" point of aim with point of impact?
All my patterning I've ever done has been with a dirty barrel, at least after the first shot. And, I've never seen one smidgen of difference.
One ounce of shot inside of 30 yards, with the bird centered in the pattern, should be a dead bird, regardless the gauge, if the shot size is sufficient and you're not getting blown patterns for some reason..
