Why a British helicopter door gun would probably use 7.62 mm ammunition

What calibre would the British services most likely use for a helicopter door gun?

Probably 7.62 mm, because the door gun would normally be the standard machine gun already in service.

The earlier helicopter-armament question has a likely calibre. A flexible door gun would normally use the standard machine gun already available across the services, recalled here as 7.62 mm. That reinforces the procurement logic behind Royal Navy adoption instead of pointing to a bespoke naval cartridge. The calibre is offered from memory rather than as part of a detailed weapons survey.