Lewis and Clark expedition did have pellet rifles made in Austria. The rifles were powered by a large compressed air cylinder which was part of the rifle. The rifles could shoot about 20x from one pump up. The problem was that it took a huge number of pumps to get to full charge. Stories are that Lewis would show off the fire power to the Indians and that kept them from even thinking of giving the expedition any trouble.
The Girandoni Air Rifle was an airgun designed by Tyrolian inventor Bartholomäus Girandoni circa 1779. The weapon was also known as the Windbüchse (wind rifle in German). One of the rifles more famous associations is its purported use on the Lewis and Clark Expedition to explore and map the western part of North America in the early 1800s.