Over ten years ago I did some extensive testing involving five different shotguns and three brands of 2-3/4" 00B shells (9 pellet). What I learned is the extreme spreads at 15 meters were all over the place and I couldn't make any generalities or conclusions. The usual generality is that it will be 1" per yard or less.

I have found that in my personal short-barreled 12 ga. pump (a Stevens 67A with barrel cut to 18-1/2") that Remington #4 Buck patterns better than 00 Buck. And that's what I use.