375 H&H Double Rifle - whilst a flanged or rimmed cartridge is very simple in a double rifle, and most double rifles are built for rimmed cartridges, plenty have been built for rimless cartridges, and even rebatted rimmed 425 Westley Richards. Some of them are ejectors.
Westley Richards developed quite an ingenious system for the ejectors where they have a blade goes up under the rim as the rifle is closed. On opening this lifts the case away from the chamber breaking an seal with chamber wall and then drops away as the ejector kicks the cartridge out. A tapered case would also help.
On rimmless break actions I have seen - there is just a spring detent which you have to pull the empty over.
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