I have a 870 SM with a 26" barrel. When shooting 3" Longbeard 5's & 6's through a .660 & .665 chokes(not IC chokes), I get good patterns at 40yds. My 20yd pattern is about the size of softball....way to tight for my liking. With a Carlson .670 non-ported tube I still get a decent pattern at 40, but my 20yd pattern opens up to about the size of a volleyball. I get better numbers at 40 with tighter chokes, but prefer the more open pattern at 20...90% of shots are under 30 yards.
I get better numbers with 6's over the 5's at 40, but a more even /less holes in the pattern with the 5's. I think I'm shooting the 1 3/4oz loads.
If you want a really tight pattern at 40, find a Remington Ventilator choke(.655) with Longbeards. At you will have about a tennis ball size pattern.