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Well obviously bobcat doesnt know what he is talking about.http://shotshell.drundel.com/pelletcount.htmThe only difference is pellet count. I shoot a bunch of geese every year with 12 ga 2 3/4 in steel shells. Its very similar to a 20ga 3" shell.
Gauge does matter. 12 gauge is better all the way around. More shot, and/or more velocity, along with a shorter shot column, which provides better patterns than the smaller gauge shotguns. Sounds to me like you need to do more homework, Professor.