This is something I did not know. I own what I thought to be a very powerful air rifle - an RWS .17 caliber single-pump model that propels the tiny pellet at about 1100 feet per second. It is powerful enough to put meat on the table if you are partial to squirrels, rabbits and small birds. What I didn't know is that much more powerful air rifles have been around for over two hundred years and were carried by Lewis and Clark on their historic trek across America. In fact, it is mentioned in the first entry into their journal where its accidental discharge struck a woman in the head.
The Girandoni air rifle was in service with the Austrian army from 1780 to around 1815. The rifle was 4 ft (1.2 m) long and weighed 10 pounds (4.5 kg). It fired a .46 caliber ball at a velocity similar to that of a modern .45 ACP!! It had a tubular, gravity-fed magazine with a capacity of 20 balls.
Here is a very interesting piece on the weapon from the National Firearms Museum: