ORIGINAL: riverboss
the backboreof a barrel has as much to do with the size of the shot pattern as the choke. most 21in barrel guns have a larger back bore so that they shoot the same size pattern as a 30in they are shorter in length to make them easier to manouver around in the brush that is why most bird hunters shoot spreeder wads to get a bigger pattern the size of the pattern might be 1/2in bigger from a shorter barrel but not enough to make a noticable difference. most short barreled turkey guns will shoot tighter than 30 in barrel gun because of there back bore.
sounds like a good theory but still not totally true. like you said IF the barrel is backbored. look at remington, the barrels are not backbored and the 21" turkey guns are the same ID as the 28" duck guns.
another factor is your speed. while modern powders are fast burning compared to many years ago there is still a bit of velocity difference between a short barrel and a long barrel gun. the velocity will effect your pattern esp when you reach that super/subsonic breaking point.
while we are at it why not through in humidity and elevation???my gun shot in new orleans will shoot tighter then yours in montana.