Was at Bisley a few years ago and met a member of the RN RM Rifle association, he offered to get me into it . Its a bit of a closed shop --
Target Rifle seems to be the norm and other types of shooting disciplines aren't really catered for.
May have changed since then but that was the impression I got.
Sierra_Hotel, keep watching the RNRMRA website (RNRMRA.org), and get in touch when you're serving. If you know anyone who shoots target rifle and is joining the RN, please ask them to make contact via the webite.