Well, we would suggest that you try Forster, as they make oversized mandrels just for such a issue. Also we would like to add, annealing and proper case neck preparation also helps. Add to that, the Forster Ultra Micrometer dies which seat extremely good too. No runout and always to the correct seating depth, with .001" accuracy. Contact Scott at Forster. Real nice guy and he will help you for sure.