I have a BPS with the 26" barrel, it's an Inv+ gun.
I've tried several choke/shell combos and the .643 SSX choke has worked best for me. It has patterned 3" Hevi-13 #6, 3" Old Federal #5 and #6 lead, and 3" Winchester 1 7/8 oz #5 lead loads really, really well. A .670 Pure Gold tempest choke did very well with the 3" Hevi-13 #6, 3" Old Federal #5 and #6 lead loads as well.
Hope this helps!