The Saxby & Palmer Ensign rifle was designed to work with a large gas cartridge system, similar in size to a 12g shotgun shell. The cartridge would be filled with compressed air, and a pellet placed in the cap.
To the best of my knowledge, only 100 of these were ever made, and mine is #87.
Sadly these are now banned under UK firearm laws, and they are classified in the same category as fully-automatic machine guns and grenade launchers... which is bloody ridiculous as it's mostly made from cast metal. It could NEVER be converted to fire anything other than the cartridges it was designed to use.
ps. This one is owned legally as I was in possession of it before the ban came in to force, so I was allowed to keep it on a firearms licence. However, I can never sell it, give it away, lend it to anyone, or export it, so I am effectively stuck with it for life.