There is a 9.3 Rigby - the 366 DGW. It was developed by a gunsmith in Maine. I'd look at some of the standardized 9.3 cartridges:
9.3x64 Brenneke - Definitely is on par with the 375 H&H in terms of performance. Brass is scarce and pricey and it's got that funny rim dimension but a fantastic cartridge.
9.3x66 Sako (aka 370 Sako Mag.) - Reaches the lower end of 375 H&H performance. Standard (.30-06) rim dimension and is available in the U.S. now - Sako 85.
