This won't help you, but I bought some .257 Weatherby PMC brass several years ago and it is by far the strongest brass available in that chambering. The cases will take loads that cause a Norma or Weatherby case to stick tight in the chamber and eject them like a light factory load. It is also dimensionally great, a better fit than Norma. I doubt that all PMC brass is that good, but I do not know.
.