Not to bend your dream but I agree with SeekHer that a rimmed cartridge would save a lot of complexity and cost versus adding a rimless extractor to an existing action. Lots of Cape Guns were in common calibres like .303 British and 8x57R. I would tend to an American cartridge like .30-40 Krag in the same ballistic category. If I wanted to feel upper crust, I would call it a .30 Purdey.
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