OK, I've spotted it at last (hint: it you want to catch my attention, put "gun" or "ammo" in the thread title - I don't have time to trawl through everything!)
Yes, it's a British Aden gun round. The RG76 means it was made at the Radway Green ordnance factory (not far from where I live) in 1976. It could have come from just about anything as Adens were used in a huge number of planes, but at the moment they remain in the RN's Harriers (not the RAF's) and the RAF's Jaguars.
No need to worry about it being live. The driving band has been engraved by the rifling, which measn the shell has been fired and recovered.
Incidentally, that round base to the shell means it was still a copy of the original German Minengeschoss, which introduced this feature. The Aden was of course closely modelled on the Mauser MK 213 revolver cannon developed at the end of the war.
Tony Williams
Author: "Rapid Fire: The development of automatic cannon, heavy machine guns and their ammunition for armies, navies and air forces"
Details on my military gun and ammunition website:
http://www.quarry.nildram.co.ukMilitary gun and ammunition discussion forum:
http://forums.delphiforums.com/autogun/messages/