At the time I built it, the only rimmed Mauser type cases available were Norma 8X57R, so that's what I used, employing a set of RCBS forming dies made to form .250-3000 from .30-'06. Today I might use 7X57R.

I have seen High Walls with rimless extractors, but they didn't appear very effective to me, and cases are sometimes hard to extract from a High Wall varmint rifle.