DoubleD:
I think you are being a little hasty in discounting the .375 Flanged Magnum: its twice the cartridge compared with the 2 1/2. I know its more about back-thrust, but the 2 1/2 inch cartridge runs at higher pressure than the Flanged Magnum. Many Martinis were re-chambered for the .375 Flanged Magnum a century ago, all were subjected to British proof loads at the time, obviously most survived proof or the practice ould hardly have been economically viable! Many of these conversions are still in use today.

I think the .375 Flanged Magnum is probably the perfect choice.

...although one could admit to bold ecccentricity and have a closer look at the 20-bore/.577 Express!!!