My Mossberg 500 (28" barrel) likes a .670 choke - something like a Mossberg factory .670 or a Carlsons will do.

If you're going to buy factory shells, get you some Hevi-13 3" 2 oz #6s.

If you want the best performance, you'll have to handload your own TSS. With that, I like a .690 sporting clays choke.