I hunt turkey, rabbit, grouse with my 870 and a 21" bbl. Works great. My buddy has a Coach Gun that excels in thick brush
It has more to do with choke than barrel length.
As for clays, I usaully take my Ruger Red Label. but wehn using the 870, I usually stick with the 21" tube. The only thing I use my 28" bbl for is Goose and Duck
NRA Life Member, SAF contributor.