Lewis had an air rifle. They used it often to dazzle the Indians. From a website...
“Lewis’s air gun was a remarkable property of the expedition. Widespread agreement is that it was a Girandoni-style air rifle, originally designed and built in Europe for the Austrian army. The Girandoni was .46 caliber, had a magazine for 20 shots, and was a repeater. Lewis used it to impress Indians in council. It was smokeless, and could fire 20 shots in one minute. The Indians were impressed. The air gun was the most unusual piece of equipment on the expedition. Air guns of that period and caliber are not silent and make a very loud crack.”
That was an amazingly advanced weapon for that time. I wonder though if they used that kind of weapon in their first (and only) attempt to hunt grizzlies, if a riffle like that simply had too low a velocity to effectively penetrate the heavy coat of a grizzly.