Why a Royal Navy helicopter gun would follow other services

If Britain adopted the 7 mm EM-2 rifle, would Royal Navy helicopters use that cartridge or a heavier gun?

The Navy would probably adopt whatever suitable helicopter weapon the Army or Air Force already had rather than fund a separate naval system.

The Royal Navy would probably not develop a bespoke helicopter gun around the EM-2 cartridge. The service would usually select a weapon already available through the Army or Air Force. This procurement pattern mattered more than a theoretically ideal naval cartridge.