Are you referring to the Lewis and Clarke expedition into the Louisiana territory after the United States purchased it from France?

Yes, he is. M. Lewis purchased a Girandoni-style repeater air rifle in Philadelphia just prior to his expedition. It is mentioned many times in his journals; however, it is never properly described in those journals. It wasn't until the journals of Thomas Rodney were recently published that we got a proper description and we were able to conclusively determine the type of air rifle he had.

Beeman asserts that Lewis's airgun must have been one produced by Girandoni himself in Vienna. But that is extremely unlikely for a number of reasons. The most likely source of the Lewis air rifle was London, which was the major production center for airguns in the era and was, of course, very active in exporting manufactured goods to America at the time. It would have been an extremely difficult and unusual trip for a Vienna-made airgun to have made it's way to America at the time.