I've found that a 1 ounce load is pretty deadly on wild pheasants be it from a 20, 16 or 12. A light well fitted gun will put you on birds faster giving you a yardage advantage over a shooter with a heavier weight shotgun and load. I have/had 12's, 16's and 20's all within a couple ounces +/- of 6# couldn't tell the difference in performance in the field.

After the first shot the rest are just noise.