Man that would be a sweet rifle! It's been years but as I recall the ballistics were 250 gr @ 2300-2400 fps. With a double it will come down to what the rifle is regulated for or rather working up loads that regulate.

It's long gone, but I built a 350 Rigby on a VZ-24 mauser and formed brass by taking 375 H&H brass, chucking it in the lathe to cut off the belt, deepen the extractor groove and reduce the rim diameter. Then through the FL sizer, back in the lathe to trim to length and hopefully the 45 deg shoulder set back far enough to allow the case to chamber and fireform. Bertram made brass, but it was soft and showed pressure signs when loaded to the relatively mild 250 gr @ ~2400 fps. The H&H brass showed no pressure signs and the gun was most accurate with 250 gr @ 2700 fps.

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There is something about those long gracefully tapered cases from the turn of the past century that just looks right.