I am always amused at the comments that a smaller gauge with a smaller shot charge will perform just as well as the next larger gauge and a larger shot charge. That line of reasoning would, by extension, say that a 28 performs as well on game as a 20, and a twenty performs as well on game as a 16, and a 16 performs as well on game as a 12, ad infinitum. If all that were true then a 28 would perform as well as a 12. Oh, really?
I'm in the court with 2-piper on this issue. He stated that the twenty will always be a slightly more effective and versatile gauge (than the 28). I agree, and would go further in saying that the next larger gauge will always be slightly more efficient and versatile as long as handling and target acquisition does not suffer. A larger bore will most always pattern better with the same shot charge than the smaller. Better patterns mean increased range capability, as long as pellet energy and pattern density are adequate for dispatch of the bird. The bigger question is, can a shooter UTILIZE the increased efficiency, small though it may be.
Fact is, we have a wonderful choice of bore sizes to use at our chosen sports, and we should use whatever we enjoy most, as long as it is efficient at dispatching game and not increasing the likelihood of wounding birds.