I recently visited the Cody Firearms Museum, in Cody Wyoming. They have the actual air spoon rifle that Lewis and Clark carried on the trip down the Columbia River. It looks exactly like your buddy's. Lewis and Clark used it mostly as a demonstration of technology for the Native American tribes they encountered along the way, but said it could bring down a deer if within it's range of accuracy.

How was the compressed air generated?

He said there was a set of bellows of some kind. I don't believe he has them, or maybe they were too rough to show - can't remember.

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It was a large wooden box or bellows with a leather diaphragm. The early ones were done by hand, but later by steam. Not too easy to carry around on a hunting trip, and probably why they never caught on. I think the one Lewis and Clark had pushed the lead ball about 1000 fps. That was faster than the black powder rounds of the day, and faster than the early 45 Long Colt.

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There's a furniture store here in Houston that has a bunch of that sort of stuff. And besides having stuff out of Al, I think it's also got stuff out of copper with brass rivets IIRC.

Girandoni Air Rifle

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357f07ee876ed048529300711c6ac263 - Ztona
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