When Lewis and Clark headed up the Missouri River with the Corp of Discovery in 1803, they became the best armed war party in the American West. One gun they took with them was an air gun. This airgun had an air tank in its stock, which would be pressurized for a number of shots. The rifle shot a lead ball just like the flintlock rifles the party carried, of .463 caliber, according to Dr. Beeman.
Photos from Vintage Airguns, thanks to Mr. Peter Nolan. Many people believe the Lewis and Clark air gun was a Girandoni airgun, with Peter's Girandoni visible here. The stock would be removed from the rifle, and be pumped with a LARGE number of strokes to pressurize the tank, and the stock could then be reattached to the rifle. Spare stocks would be carried which could each be fully charged. The air gun was a repeater, and would shoot multiple rounds before the balls and air had to be replenished. It had the external appearance of a typical flintlock, but the stock was more rounded.
This air rifle became a centerpiece in the Captains' dog and pony show, and was fired at every meeting and pow wow with the Indian tribes met along the way. The intent was to impress on the Indians the great technology held by the Americans, and to thus foster cooperation. The gun was demonstrated in a way that hid the technology of the gun, with the Indians deliberately left with the impression that it was powered by magic.
MUCH more information on air guns is to be found at the Henry Stuart Antique Firearms site, of the VMI Museum.
The gun was described in the journal of a traveler (Thomas Rodney) on the Ohio river who met Lewis, and saw it demonstrated.
“Visited Captain Lewess barge. He shewed us his air gun which fired 22 times at one charge. He shewed us the mode of charging her and then loaded with 12 balls which he intended to fire one at a time; but she by some means lost the whole charge of air at the first fire. He charged her again and then she fired twice. He then found the cause and in some measure prevented the airs escaping, and then she fired seven times; but when in perfect order she fires 22 times in a minute. All the balls are put at once into a short side barrel and are then droped into the chamber of the gun one at a time by moving a spring; and when the triger is pulled just so much air escapes out of the air bag which forms the britch of the gun as serves for one ball. It is a curious peice of workmanship not easily discribed and therefore I omit attempting it.”
Animated views of the firearm and and its firing (three seperate animations) are to be found at the Animations page of the Discovering Lewis and Clark site, along with much more information about the Core of Discovery.
The air gun was described as being "of the Girandoni type", referring to an Austrian maker of similar air guns, although some believe that it was a Girandoni, sold by a U.S. dealer. More information and illustrations on the Lewis air gun and Girandoni type air guns are on the site of Robert D. Beeman PhD.