The 350 rigby is a unique case, but it can be formed by turning the belt off of a 375 H&H case, turning down the rim, cutting a deeper extractor groove, and fl sizing the case to set the shoulder back, then trim to length. Bertram makes 350 rigby brass but it's soft, Hornbear supposedly does but I never found anyone that had it in stock. Forming cases from H&H hulls ia a PITA, but they work fairly well. The 350 rigby pre-dated the 375 H&H by 2 years.
Rimmed would be easier as all you'd need to do would be run a 375 flanged case into the fl sizer.
Somewhere I have a picture of the 350 rem mag, 35 ai whelen and 350 rigby lined up. I sold my 350 rigby this summer, the 350 rem mag is a much better balanced package, lighter gun and all I had to do was give up 25 gr of bullet for the same velocity and sub moa accuracy.
