Editing Air gun (section)

=== Compressed gas ===
[[File:BB gun with CO2 and BBs.jpg|thumb|right|200px|CO<sub>2</sub> pistol and disposable [[Powerlet]] cylinders]]
[[File:Nitrogen tank.jpg|thumb|250px|A Pure Energy N<sub>2</sub> tank with a remote line attached]]
'''Compressed gas guns''', also known as ''CG guns,''<ref>{{cite web|url=http://www.pyramydair.com/article/What_About_CO2_December_2003/6|title={{CO2}} is used to power many air guns|publisher=PyramydAir|author=Tom Gaylord|access-date=15 January 2017}}</ref> are essentially pneumatic airguns utilizing detachable [[pressure vessel|pressure reservoir]]s in the form of prefilled external [[gas cylinder]]s (often with built-in [[pressure regulator|regulator valve]]s), and are commonly referred to as ''CO<sub>2</sub> guns'' due to the ubiquitous commercial use of [[bottled gas|bottled]] [[liquid carbon dioxide]].  However, more recent high-end models sometimes use larger [[compressed air]]/[[nitrogen]] (known as HPA or "N<sub>2</sub>") cylinders, which have higher operating pressures and better thermal stability.

==== CO<sub>2</sub> ====
[[File:Walther CP88 Schall.jpg|thumb|[[Walther CP 88]] CO<sub>2</sub> pistol with adapted silencer]]
[[File:Crossman-2240.jpg|thumb|Crosman 2240 CO<sub>2</sub> one shot pistol, (.22 pellet caliber)]]
'''CO<sub>2</sub> guns''', like other pneumatic guns using compressed air, offer power for repeated shots in a compact package without the need for complex pumping or filling mechanisms. The ability to store power for repeated shots also means that repeating arms are possible. There are many replica revolvers and semi-automatic pistols on the market that use CO<sub>2</sub> power.  Most CO<sub>2</sub> guns use a disposable cylinder called a "[[Powerlet]]" cartridge, that is often purchased with {{convert|12|g|oz}} of pressurized CO<sub>2</sub> gas, although some, usually more expensive models, use larger refillable CO<sub>2</sub> reservoirs like those typically used with [[paintball marker]]s.

CO<sub>2</sub> guns are popular for training, as the guns and ammunition are inexpensive, relatively safe to use, and no specialized facilities are needed for safety. In addition, they can be purchased and owned in areas where firearms possession is either strictly controlled or banned outright. Most CO<sub>2</sub> powered guns are relatively inexpensive, and there are a few precision target guns available that use CO<sub>2</sub>.

==== High-pressure air ====
[[High-pressure air]] (HPA) system, or ''N<sub>2</sub>'', was originally developed for [[paintball markers]] as a replacement for CO<sub>2</sub> cartridges, and uses Powerlet interface-compatible [[diving cylinder]]s filled with either pure [[nitrogen]] or [[compressed air]] (which is 78% nitrogen). Because nitrogen is more [[chemically inert|inert]] than CO<sub>2</sub>, it remains in the [[gas]]eous state longer when compressed; when it expands, it cools due to the [[Joule-Thomson effect]] but at a far lower rate than [[liquid carbon dioxide|liquid CO<sub>2</sub>]] because there is no change in [[state of matter|state]] [[boiling|from liquid to gas]]. This [[thermostability]] reduces the variation in output pressure associated with rapid successions of firing cycles, improving accuracy and reliability in extreme climates. However, because compressed air is stored at higher pressures (up to {{cvt|34|MPa|disp=or|lk=on}}) than liquid CO<sub>2</sub> (stored at around {{cvt|8|MPa|disp=or}}), HPA cylinders are more expensive. Cylinders smaller than {{cvt|1.1|L|cuin|lk=on}} may not even last as long as a standard CO<sub>2</sub> cartridge when subjected to frequent uses.

It is also possible to power an HPA-compatible gun directly from a plug-in [[air compressor]] without using a gas cylinder, though this type of setup is rare.

====Air cartridge system====
First developed in the [[United Kingdom]] during the 1980s as the Saxby-Palmer Ensign,<ref>{{cite web|url=https://www.pyramydair.com/blog/2006/01/brocock-air-cartridges/|title=Brocock air cartridges|last=Gaylord|first=Tom|date=18 January 2006|publisher=Pyramyd Air Blog|access-date=25 July 2021}}</ref> and then by Brocock as the "Brocock Air Cartridge System" (BACS) and later into the "Tandem Air Cartridge" (TAC),<ref name="BACS">{{cite web|url=https://www.airgunmagazine.co.uk/features/air-cartridge-system-10-years-later/|title=The air cartridge system – 10 years on|last=Cross|first=Rupert|date=1 September 2013|publisher=Airgun Shooter Magazine|access-date=25 July 2021}}</ref> this variation of the pre-charged pneumatic design have no pressure reservoir built into the gun, but instead use removable and reusable metallic [[gas cylinder]]s often known as "Brocock cartridges" after its British manufacturer.  Each air cartridge is essentially a self-contained gas reservoir housed inside a [[cartridge case]] (usually in the size of a [[.38 Special]]), with an internal [[bash valve]] designed to release the content when the base of the cartridge is struck. Prior to shooting, each cartridge is pre-filled with sufficient [[compressed air]] for one shot (typically via a multi-stroke [[hand pump]]) and has a [[.177 caliber|.177]]/[[.22 caliber]] pellet pre-seated to its front slot, and the entire cartridge is then inserted into a [[chamber (firearms)|chamber]] in the [[gun barrel]]. When shooting, the [[hammer (firearms)|hammer]] hits the rear of the air cartridge, mimicking the function of a [[centerfire ammunition]]. This allows the airgun to be constructed and operated in similar fashions to genuine firearms. It also allows for higher shot consistency because each cartridge can be easily filled to an identical air charge, essentially removing the "power curve" of conventional PCP guns and bypassing any need for [[pressure regulator|regulator]]s. It also simplifies the magazine feed design and eliminates the risk of deforming the projectiles when the [[action (firearms)|action]] seats each pellet, as the soft lead pellet is protected from contact with the action probe by the harder cartridge casing.

The air cartridge system, both in the [[rifle]] and [[revolver]] forms, was at the peak of its popularity throughout the 1990s. The popularity of the Brocock range was such that, by 2002, estimates put the number of air cartridge guns in circulation around 70,000-80,000.<ref name="BACS"/> However, a small number of incidents relating to the alleged illegal conversion of (mainly) Brococks to [[improvised firearms|allow them to discharge live ammunition]] sparked a [[media frenzy]]. In early 2002, the [[BBC]] reported that figures from the [[National Criminal Intelligence Service]] showed converted Brococks accounted for 35 per cent of all guns recovered by the police,<ref>{{cite web|url=http://news.bbc.co.uk/2/hi/uk_news/england/1779349.stm|title=Live-bullet airgun a 'major problem'|last=|first=|date=24 January 2002|publisher=BBC News|access-date=25 July 2021}}</ref> and David McCrone, firearms adviser to the Association of Chief Police Officers and Deputy Chief Constable of [[Greater Manchester Police]], told the BBC's ''[[Newsnight]]'' "there is evidence which would justify banning them". After the [[UK government]] implemented the [[Anti-Social Behaviour Act 2003]] in January 2004, it became illegal to possess such airguns without a [[firearm certificate]] in the United Kingdom.  Brocock subsequently ceased production of all air cartridge systems and turned to focus primarily on PCP airguns.