I suggested fireforming a 6 m/m Rem case several days ago. I expect it will blow out to .257 Roberts dimensions, or some improved version thereof. Use new or annealed brass so it fills out nicely without any splits. Of course, using .257 brass for your fireform load will work too. I suggested 6 m/m because it sounded like you already had that cartridge on hand.
The .257 Roberts is a great cartridge for your wife to use, so hopefully you'll get the standardized version that is available in off-the-shelf reloading dies. Good luck.
