RE: .257 Weatherby Magnum
savagescout, the guy I mentioned above was told the same thing by a veteran reloader. He fire formed some rem mag brass buy using a small load of fast burning pistol powder and patch in the neckto create a seal. Thecases he showed me looked spot on but hasn't tried themunder load yetto my knowledge.
Sorry for the second hand info, hopefully someone with hands on will chime in on the reloading forum.