The American Rifleman had an article years ago on the Lewis and Clark expedition's weapons, and supposedly, they had an air rifle with them. They were extremely cumbersome back then. From what I have read, the modern ones are extremely powerful in the larger calibers. It was always said that any of the pellet gun manufacturers could easily make a .22 caliber pellet rifle as powerful as a .22 Long Rifle, but they do not, for fear of the anti-gunners getting their panties in a wad over them, and then wanting to impose regulations and what not on them as they do regular firearms. MM