I have the same gun, but 22" barrel.
I shoot a Jellyhead .660 with Hevi 13 in the 3" 2oz #7 load. I'd start with that factory X-full choke that came with it, I mic'd mine at .666 with a calibrated mic and run the Winchester LB shells in #5 through it, see what you get.
Good luck, keep us posted. Like your user name by the way.