RE: Choke/Ammo Combo for 870
You can also call up Carlson's and they can do a very good job with outfitting you with a choke...VERY knowledgable staff that works there and they are more than happy to help. My dad is a trapshooter and they have helped him over the years with chokes in his shotguns.
I personally use the Remington Hevi Shot choke paired with the Hevi Shot ammo for my 870...it just works well for me, so I stuck with it