Check out Trulock chokes, they make a good extended choke with lifetime warranty and aren't as expensive as many others. I have a mod on my 870 and it patterns very well. For the longer range you might want to try the imp-mod. My Trulock mod shot similar in density to the factory choke but with much more consistent pellet spread. Basically there weren't as many holes or clumps of pellets.

Check out Sportsmans guide, only $26. http://www.sportsmansguide.com/cb/cb.asp?a=71275