If Lewis and Clark had a Girandoni (alternate theory is their airgun was made by a Philadelphia maker), it was Belgium army surplus. Girandoni built them for the Belgium army and were pretty much standard issue.
Air reservoir was in the butt stock. Separate pump kit took 1500 strokes to pump each butt stock, each soldier was to have 3 charged and something like 100 lead balls ready for firing at any time. Eventually, the army invested in pump trucks to charge reservoirs as needed. All powered by hand of course. I was told once that if a soldier was caught with a Girandoni during the Napoleonic wars he was immediately shot as a sniper.
Imagine an army using muzzle loaders. Firing in ranks. Each soldier kneels, volley fires, stands, moves to the back of the line, performs a reload, and then does it all over again. The smoke would give any lone sniper away.
Now, imagine an army fitted with Girandoni rifles. 24 ball magazine. All you had to do was point the gun up, press the spring loaded button to drop a ball into the chamber, cock the gun, and fire. You could do that from a prone position with no smoke and no noise.
Sadly, manufacturing techniques were not up to the task Brazing the butt stocks together to hold air was problematic, they were high maintenance, and as black powder got more powerful and cartridge's were introduced, the advantages were lost.
Rumor has it that Lewis and Clark had one with them on the Corps of Discovery. They'd use it to impress the native inhabitants with a "repeater" rifle.
Airgun history goes way way back.
At least, that's the story as I know it.
