Any double rifle cartridge needs to keep pressure in check if one expects it to last. 20 tons is the absolute maximum, 15 tons is much better. With that in mind one should go for a medium caliber and a large case.

I’ve have the opportunity to experiment with my .375 H&H. Stepping down to a 235gr bullet, using H414 powder and finding the slowest regulation load – I have reduced the pressure way back.

Based on that, I reckon that a 300 UM case necked up to .350 shooting a 220 grainer at 2700 fps might fit the bill.