Centaur,
I'd suggest that you go over to the National Wild Turkey Federation site and read about the 20's in the "shotgunning" section. Seems a lot of guys there hunt with them and can help you out especially a guy who calls himself "allaboutshooting". They are shooting out to 35 yds. or greater. I have 20 ga 870 with a .575 Rhino choke and use Nitro Company shells (4x5x7 hevi) and I can easily take a bird at 40 yds. Most of the time though I use a 12 ga 870 with Nitros and Rhino choke.

Jim