Holmes edged his way round the wall, and flinging the shutters togehter, he bolted them securely.
"You are afraid of something?" I asked
"Well, I am."
"Of what?"
"Of air-guns"— "The Final Problem" by A. Conan Doyle
Basic Information
A airgun is a weapon, similar to a firearm but which uses mechanically compressed gas - usually compressed air - to fire its ammunition. They are normally made in pistol or rifle configurations, although some smoothbore types exist as well.
In the modern era, airguns are something of a sideshow - at best they tend to be low powered hunting weapons, generally of .177 or .22 calibre and designed for taking small game and vermin. More generally they can be used for basic level skill at arms training1 or as toys. That said, modifiied versions of one of the toys - the paintball gun - are used by some police forces to mark suspects for arrest and/or to deliver pellets of skunk oil or CS gas (or substitutes for either). Compressed gas is also frequently used to propel hypodermic rounds - tranquiliser darts and the like - which would probably not stand up to being discharged from a firearm.
In the pre-modern period, by contrast, a compressed air powered weapon was a reasonable competitor to contemporary firearms and several experimental weapons were developed, probably the best of which was the Girandoni Windbusche which can arguably lay claim to being the world's first repeating rifle and with decent accuracy and hitting power and a rate of fire streets ahead of anything powder driven served the Austrian army from around 1780 to 1815, despite being complicated, fragile and expensive.
Rare ordnance class weapons include the Dynamite Gun and several designs of air-driven heavy mortars used by Germany during WW1.
Often airguns are lightly regulated - or not regulated at all - in all but the most oppressive regimes, but they may be subject to fairly tight design restrictions on things like magazine capacity, muzzle velocity or ability to store pressure.
Ammunition is generally loaded as loose single units - some models have a magazine but it will almost never be as complex as that for a firearm and generally takes the form of a simple hopper. The compressed gas is either supplied from a bottle (normal for toys), built up in an internal reservoir by pump action or lever action or generated by an internal spring cocked by break action or lever action.
Notable uses of air guns in fiction include the rifle used by Colonel Moran in the Sherlock Holmes story "The Adventure of the Empty House", and the underwater rifles employed by Captain Nemo in Twenty Thousand Leagues Under the Sea