Hmatt - some years ago, I did as suggested above - new PPU head stamped "8mm Mauser" brass - opened neck in first pass to 9.3 with Hornady Custom Grade dies, then to straight wall (or nearly so) with a plug that I made for a Lee "Universal Case Expanding Die". Then back into 9.3x57 die and worked shoulder back until the cartridge just barely closed in my rifle - no clue if that is to anyone's spec, but that brass fits this chamber / bolt with really minimal end-play. I could not find a Lee case length gauge tool for 9.3x57, so made one by shortening one for 9.3x62. Of course, resulting cases have the original PPU "8mm Mauser" head stamp, which apparently bothers some people. I do not think I have ever seen "proper" 9.3x57 brass or ammo for sale, certainly not any place that I could buy it.
Is several other people that I know of who simply run 8x57 case into 9.3x57 die - load and fire it.