The Armour Archive

http://www.liveleak.com/view?i=0b8_1296099360

It is a very long monograph about it, and they apparently made a reproduction. At the very bottom is a slideshow of an original, and diagrams on the workings.

Ah the Girandoni air-rifle. I love those things. Check out the Beeman Airguns page on the Girandoni design. It was so good that the French resistance used them. Anyone with access to a shop that can manufacture a bicycle can make one. I've got a couple of designs that I'm considering working on once I get some time cleared up. It's funny. If you add a follower spring to the magazine then you don't have to tip the rifle up to gravity load the next round and the Girandoni becomes, for it's time, a very fast weapon with no real signature. Just one spring and the other barriers (more complex maintenance and the higher degree of training required to maintain and operate the weapon) become a worthwhile trade off.

Don't have the technical skill to do that secondary spring (mechanical idiot here), but that sounds like a GREAT addition for squirrel-hunting.