Air guns are, as a general rule, guns which use some mechanism involving air or carbon dioxide to launch a projectile at a target. Air guns can be used for much the same purposes as firearms, but usually at shorter ranges. They tend to be popular in jurisdictions such as the United Kingdom and parts of Europe, where actual firearms are heavily restricted or banned outright.
Air guns can use two different types of ammunition, pellets, which are shaped like standard rifle projectiles. This means they have a cavity at the bottom to expand and meet the rifling. Usually made of lead, but certain types of pellet, such as 'Prometheus' pellets, are made of different materials which expand in the barrel, but do not flatten on impact. Available in a range of calibres, but .177/4.5mm and .22/5.6mm are most common. Ball-bearings can be made of steel or lead and are only available in .177 (for the purposes of this article)
!'''Types of air guns'''
* Spring-powered: Use a spring to compress the air. Almost always single-shot, but there are exceptions. Often referred to as recoiling for competitions.
** Break-Barrel: The barrel is swung downwards to load the round.
** Under-lever: A lever under the barrel cocks the spring and opens the chamber.
** Lever-action: Replicas of Winchester-type lever-action rifles.
* PCP (Pre-Charged Pnumatic): These air guns use a tank of compressed air to fire the pellet (and it is always a pellet with a PCP).
* CO2: Similar to the above, but uses a canister of CO2 instead. Generally intermediate in power. These are often used in gas-blowback mechanisms, which use the CO2 to imitate a semi-automatic pistol or rifle.
!'''Air gun laws'''
* In the UK, where air guns are more popular than real firearms, they do not (usually- see below) require a license but can only be purchased from a registered firearms dealer by someone aged 18 or over and their use is subject to similar laws as real firearms. Scotland has recently announced plans to implement a license for air guns.
** Air rifles with a power above 12ft/lb or 16.3J require an FAC (Firearms Certificate) to own, so you may as well get an actual rifle. Air pistols over 6ft/lb or 8.1J are banned outright.
* The US: Areas that have tight restrictions on firearms are likely to be more lenient when it comes to air guns. Just in case, however, ask the local police or your lawyer for advice.
** Just beware. Many states' definitions of firearms do include airguns, even if it's rarely enforced as such. Look for the words pneumatic and calibers down to .177/4.5mm. It probably will never be an issue if you're not doing anything stupid, but it's worth keeping in mind. New Jersey is a major exception to this, with some air rifles even fitting the state's definition of "assault weapon."