It was a 300wsm, and so I think I used 9/16” or thereabouts. Usually I try to find a flex hone just slightly larger than the part I’m polishing internally. Ideally you’d get a 320 then a 600, but I was being cheap so I did extra with the 600 and some oil.What width flex hone did you use on the die? I've got the same problem with a Hornady die.