you can get bore size backs like evil sly etc but these kits are expensive ? freak kits rely on different insert that go into a back can get inserts for £5 each and a back £10 ? or complete set around £40 if that helps
I find a 688-689 barrel works on my pumps for all the paint I currently shoot, and rarely need to go any smaller than 685. I would keep your eye out for a cheap freak set, or barrel kit around those sizes.