David,

I'm not an expert, nor understand the science of it. But I believe for double rifles, there are two issues. Pressure and back thrust (?). Two cartridges might have similar pressure levels, even if one is a .300 and one a .458. However a big bore will have more back thrust. Which is a factor for the relatively weaker break open DR actions.

Something to bear in mind.

Now if anyone wants to tell me the above is crap with sensible constructive factual argument, go for it. Or explain it better than me.