An air rifle is a device possessed of a rifled barrel, designed to accurately project pellets over a distance, the motive force being compressed gas. This gas is most commonly air but can also be
cardon dioxide and sometimes
freon. Insofar as a
firearm uses the pressure of expanding gas to propel bullets, an air rifle is fundamentally no different from any other kind of rifle; however, the volume of gas produced by gunpowder and its heirs is much greater than the amount of air which can be realistically stored in the body of an air rifle, and thus such rifles are usually of lesser power than firearms.
This is not an absolute rule. If one discounts blow guns, air rifles date from the 16th century, originally for hunting medium game - wolves, small bears and so forth. Although less powerful than contemporary muskets, they were easier to load and fire repeatedly, and produced less noise and smoke. Despite the cost and complexity of air weapons, Austria experimented with the military use of air rifles during the late 1700s, without much success; the rifles themselves were well-designed and ahead of their time, capable of rates of fire that would not be approached until the bolt action, magazine-fed guns of the late 1800s, but they were less lethal than muskets and crucially much more expensive. Whilst air rifles were expensively handcrafted with great precision, bought and used by noblemen, muskets were much easier to mass-produce, only the trigger mechanism requiring complex work.
The advent of smokeless nitro-cellulose and cordite powder in the late 1800s caused an immediate hike in the power of firearms, forevermore disqualifying the air rifle as a military arm. Nonetheless, air weapons continued as silent assassination tools for several decades afterwards, most famously in the case of Bulgarian defector Georgi Markov. Whilst waiting for a bus on Waterloo Bridge one day he was nudged in the leg by a man holding an umbrella; admitted to hospital, he died three days later of ricin poisoning, the poison traced to a metal pellet found in his thigh. Although he did not know it, the man had been a KGB agent, his umbrella a disguised air gun. Underwater spear guns are also powered with compressed gas, as are most tranquilising dart guns.
Air rifles typically fire bb balls or lead pellets, the latter being of a waisted design, with a little skirt attached to the bottom of a solid nub; the overall shape is rather like a shuttlecock. Whereas the skirt of a shuttlecock provides for aerial stability, the skirt of an air pellet performs a different function, derived from the Minie bullet of the mid-1800s. When the pellet's skirt is blasted with air it expands, gripping the rifling in the gun's barrel more tightly, sealing the barrel against escaping air and increasing efficiency. By far the most popular air rifle calibres are .22" and .177", with .20 and .25 distant seconds. Generally, the larger-calibre bullets travel slower but pack more of a punch than the smaller-calibre bullets, which in turn have a shorter flight time, subjecting them to less drag from the air and from gravity. As a side note, bb guns are generally air rifles - a bb is a single shotgun pellet - although this is not necessarily true from a historical viewpoint. Pellets are almost always metal, most commonly lead and steel. Lead is denser and more toxic than steel, which is lighter and cheaper. However, steel pellets and particularly BB balls carry very little power and tend to bounce off solid surfaces.
There are several different air rifle operating mechanisms, which divide roughly into two main groups. The most common group are 'spring-powered', also called 'springers', as they use a built-in spring to compress a single charge of air. After the spring is cocked - usually by breaking open the barrel, or with a pivoting lever - it is held until the trigger is pulled, at which point the spring drives forwards, compressing air in the rifle's reservoir. This compressed air drives the pellet onwards whilst simultaneously acting as a pneumatic cushion for the onrushing spring. For this reason springers should never be fired 'dry', without a pellet in the chamber. Such guns can fire one shot at a time, and are cheap, fairly powerful and quite accurate. Furthermore, they are self-contained units, requiring no external charging equipment. The motion of the spring as it uncompresses causes recoil, whilst minor imperfections in a break-barrel rifle's barrel lock can degrade accuracy. As the spring compresses air, the air heats up in a process similar to that which occurs in the cylinders of a diesel engine. This can cause incorrect-applied lubricating oil to ignite, in a process called 'dieseling'. Opinion is divided as to whether this increases an air rifle's power. It certainly reduces the gun's lifespan.
The second major group of air rifle designs are 'pre-charged', which is to say that they are powered with a reservoir of gas which is charged before shooting, commonly from a scuba tank or a hand pump. The gas is compressed before it enters the rifle. Consequently, when fired, the expansion of the gas actually cools the rifle down. This tends to reduce power over time, as chilled gas expands much less violently, especially so in the case of CO2-charged rifles. Many pistols are also powered with disposable bulbs, 'powerlets' as they are called, which combine the ease of use of pre-charged guns with the low cost of springers. The cylinders are much the same size as a pistol magazine, and can be stored in the gun's handgrip. (This has led to the proliferation of a subculture of imitation handguns in the UK, in particular the Brocock brand. This latter operating system combines the pellets and the air cartridges into a single unit, the unfortunate result of which is that Brocock's pistols - most commonly revolvers - can easily be converted into firing .22 and .177 firearm ammunition. Although converted Brococks are a fashion accessory amongst Britain's youth, they are poor firearms. Air pistols are not strong enough to withstand the pressures and temperatures generated by firearm ammunition, notwithstanding the low power of .22lr. Furthermore, air pistols have much weaker barrels and tighter rifling than firearms. Repeated shots - assuming the pistol does not explode - will quickly convert the pistol into an inaccurate smoothbore weapon.)
Precharged air rifles are extremely accurate and very quiet, as they have almost no moving parts. They are typically charged from a scuba tank, giving several dozen shots, or from custom-fitted stirrup pumps, which have the advantage of being cheaper in the long run. Such rifles are usually equipped with some form of magazine allowing for several shots before reloading.
Oddities including pump-action designs which are much the same as pre-charged rifles, but with a built-in pump, and 'gas spring' rifles, which are similar to springers, except that the spring compresses a sealed, gas-filled pneumatic reservoir rather than an empty chamber. Springers and pre-charged models are by far the most popular, however. Due to a century of restrictive firearms laws, air rifles are very popular in the UK, with Daystate, Air Arms, Logun and ex-firearm companies BSA and Webley producing popular designs. Other top manufacturers include America's long-running Crosman, Germany's Weihrauch, Spain's Cometa and several firearms companies, including Winchester, Steyr and the American branch of Walther. Prices range from forty pounds or thereabouts for a cheap Chinese design up into the high hundreds for the top of the range pre-charged guns, even higher for customised designs.
In the UK air rifles are restricted to a maximum power output of 12lb/ft (6lb/ft for air pistols). Apart from that they are unregulated, and popular for punching holes in vermin such as crows, rabbits, tin cans, rats and magpies and books and pressurised cans of deodorant. Silencers - often euphemistically called 'moderators' - are also unregulated, and pre-charged pneumatic airguns thus equipped are excellent for plinking without disturbing the neighbours. Airguns of a power above this are treated as firearms and regulated as such, which tends to cause shooters to choose an actual firearm instead. As a rough guide, a 12lb/ft air rifle can kill animals up to the size of a rabbit if shot through the chest or head; for the purposes of self-defence against a human being such an air rifle might blind or otherwise dissuade a human being, but would not reliably kill, or at the very least would not cause an injury that could not be treated. People have been killed by air rifle pellets - a shot in the neck or mouth would be very dangerous for a young child, and therefore air rifles should be kept and used in a safe fashion at all times - but real street gangs prefer to kill people with actual firearms, and they should know.
As an offensive weapon, air rifles would be extremely handy in a world where human beings are made out of jelly, assuming that these jelly-beings do not wear thick clothing. Unfortunately, in the real world, air rifles are not unless one welds a bayonet to the end of the barrel, in which case the rifle becomes an expensive, heavy, fragile pike.