http://www.backwoodshome.com/articles2/ayoob120.html"The 20-gauge shotgun, particularly in the gas-operated semiautomatic version, seems to be one of the best-kept secrets in the firearms world. About the only thing it's not very good for anymore, since heavy lead shot was legally forbidden for waterfowl hunting, is the harvesting of high-flying ducks and geese. In big bear country, I might also prefer the bigger slugs of a 12-gauge.
The 20-gauge will always "kick" less than the 12, and it will handle better and faster on quick-moving upland birds. With a buckshot load, the standard 20-gauge shell will fire 20 pellets of #3 buckshot (approximately .25 caliber), which will penetrate to about the same depth at about the same pattern width as the standard 12-gauge "urban load" of #4 buckshot, which comprises 27 pellets (about .23 caliber). The wound paths will be about the same depth and width, and the only difference will be how finely the macerated tissue has been chopped by the projectile paths. "Only the Medical Examiner will know for sure," and then only when he counts the little lead balls recovered from the body, or the white dots on the X-ray of the corpse."