The Girardoni (also spelled Girandoni) air rifle was a very advanced design adopted in 1780 by the Austrian Army. While the standard arm of the day was a single-shot flintlock, the Girardoni offered a massive firepower advantage to the men who carried it. The guns (designed by Bartholomäus Girardoni, of Vienna) had a magazine capacity of 22 round balls, which could all be fired within 60 seconds. The balls were .46 caliber, weighing approximately 153 grains, and were propelled at 400-450 feet per second. They were rumored to be silent, but actually had a loud report (although quieter than gunpowder firearms). One of these rifles was carried by the Lewis & Clark expedition into the American West.
The Austrian Army used them for a relatively short time - they were taken out of service by Imperial order in 1788, and issued back to Tyrolian sniper units only in 1792. The reasons for their replacement were more logistical than the result of any actual shortcoming with Girardoni's design. The problem was that they required special training to use (compared to a normal firearm), required specially trained and equipped gunsmiths to repair and maintain, and difficulty maintaining them in combat conditions. Dr. Robert Beeman has written an outstanding illustrated article on Austrian airguns in general and the Girardoni in particular, which I highly recommend for anyone interested in more detail on these fascinating weapons:
However, I am privileged to be able to share with you this video of an original 1780 Girardoni put together by Luke Haag for presentation at the 2014 AFTE conference in Seattle. Mr. Haag does a great job explaining the operation of the gun, its capabilities and accessories.