I saw the question and guessed many knowledgeable guys would answer this one.

I think there are a few issues with break open rifles, case design and pressure.

1. Rimmed or flanged. Better extraction. The reason a better choice is a rimmed bras cartridge.

2. Pressure. Pressure is an issue as the break open actions are often weaker than say a bolt action. However in my inexperienced opinion when the rifle calibre is smaller and the case smaller it is less of an issue because of no. 3.

3. Back thrust. Or whatever it is called. Combining higher pressure with greater back thrust is a problem with break open rifles. Larger calibre cartridges develop greater back thrust than smaller calibres. The reason higher pressure big bore cartridges are often less wise in a break open rifle than lower pressure alternatives. The same pressure level say in a .450 sized cartridge would be unwise.

But it seems to me, the reason the higher pressure 7x65R is fine in break open rifles, is while it has higher pressure, it is rimmed so better extraction, and the cartridge size develops lesser back thrust so the actions can handle the pressure.

Is the above incorrect? Simplistic? Correct? I am not a gunmaker or an engineer. I welcome corrections.