I tried some 2.75" and 3" #4 Buck through the 870. My 10" Circle numbers at 40 Yards were about the same with either 3" 00 Buck, or #4 Buck. Of course, the Total 20" Circle numbers were a little better with the #4 Buck at 40 Yards.
After many patterns with the 870 now, I've gotten 8-12 Pellets in 10" at 35-40 Yards, with 00 Buck, or #4 Buck.