I would try Hevi 13 #6 or #7 shot. I have a Remington 870 with 21" barrel I am going to be testing soon. I think I have a .570 I am going to be testing but it is not a Pure Gold Choke. I am not sure if Pure Gold makes 20 gauge choke in tighter constructions but I have heard that 20 gauge in .562 range was getting better patterns. Well should I say tighter patterns. Not sure how tight you are looking for and what range you plan on shooting. Depending on the shells the .570 might be the ticket for ya. Good Luck too you. Let us know what combo you come up with.