With my reload buck shots (oo) Im getting about 7" group pattern at 25 yards with my m500a1. Too bad their not in a circle pattern but more of a vertical pattern. Still fooling around with packing it to see if I can get it to pattern tighter and in a circle pattern. Not too bad for using a jig-head mold for buck shots.
A good 25 yard pattern with buckshot would be around 18" to a max of 24" with even distribution. Past that distance you'll typically start to get flyers and the pattern will most likely open dramatically and become less dense and uniform.