.660 or .665 like mentioned. I have a .660 ported in my Mossberg 535 and it's got a great pattern at 20 and 40. I have a .665 ported in my Benelli M1 and it's even better at 20 and still good at 40.
For 40 and in, I would say 665. 660 will get you a touch tighter at 20 but probably buy you a little distance. I shoot Federal 3" 9's out of both. I can send you pattern pictures of both guns at 20 and 40 if that helps, but neither is an 870, nor are mine short barrels. One is 26" and one is 28"