4seventy:
Finally got around to measuring the chamber wall thickness of my Alex Henry 20-bore/.577 Express. It's in the order of 220 thou, and damascus remember.

Even though this case is bottle-neck (like a .577/.500 on steroids!), it is only 2 3/4 inches long so it still has a similar case-volume to the .577 x 3-inch Express, and identical ballistics. I think we can safely assume it turned-up around 10 tons of pressure with the original 167gr BP load.

This would make your 235 to 240 thou of modern steel seem quite adequate. Dunno whether I'd press for the 95-100gr cordite equivalent though, it might just be pushing the pressure/ recoil/ gun-weight combo beyond comfortable limits. Having seen what the 65gr-equivalent load does to buffalo, boars, and feral horses, I think you could be quite pleasantly surprised by the all-round utility of a .577/75/650 to 577/90/650 double. The latter load generates damn near 5500 ftlbs, which is your target ME anyway.

Just be sure and let us know what you decide. You'll get plenty of heckling from the gallery either way!