I have had this thing sitting on my shelf for years. It was given to me by my father in law before he passed away. He was in the British Royal Airforce during the sixties, so I should have figured it out sooner. Anyway, I brought it to a friend last night who figured it out for me. It is a "spent" 30mm Aden Cannon round. This is the cannon round used on the RAF fighter jets during the late fifties and thru the sixties. This one I believe is a practice round. The projectile has rifling marks on it. Someone probably picked up a case and found the projectile out in the desert when my father in law was stationed in Tobruk and gave it to him. Or maybe he found them himself. Here is a video of the round:
http://www.youtube.com/watch?v=KN8EUHnpQG4
Very interesting old round... 8)