Spring-piston
Spring-piston air guns are able to achieve muzzle velocities near or greater than the speed of sound from a single stroke of a cocking lever or the barrel itself. The difficulty of the cocking stroke is usually related to the power of the gun, with higher muzzle velocities requiring greater effort.
Spring-piston guns operate by means of a coiled steel spring-loaded piston contained within a compression chamber, and separate from the barrel. Cocking the gun causes the piston assembly to compress the spring until a small hook on the rear of the piston engages the sear; pulling the trigger releases the sear and allows the spring to decompress, pushing the piston forward, thereby compressing the air in the chamber directly behind the pellet. Once the air pressure has risen enough to overcome any static friction and/or barrel restriction holding the pellet, the pellet moves forward, propelled by an expanding column of air. All this takes place in a fraction of a second, during which the air undergoes adiabatic heating to several hundred degrees and then cools as the air expands.
Spring-piston guns have a practical upper limit of 1200 ft/s (370 m/s) for .177 cal (4.5 mm) pellets. Higher velocities cause unstable pellet flight and loss of accuracy.[citation needed] Drag increases rapidly as pellets are pushed past the speed of sound, so it is generally better to increase pellet weight to keep velocities subsonic in high-powered guns. Sonic crack from the pellet as it moves with supersonic speed also makes the shot louder, sometimes making it possible to be mistaken for firearm discharge and drawing unwanted attention. Many shooters have found that velocities in the 800 - 900 ft/s (270 m/s) range offer an ideal balance between power and pellet stability.
Most spring piston guns are single-shot breech-loaders by nature, but multiple-shot guns have become more common in recent years. Spring guns are typically cocked by a mechanism requiring the gun to be hinged at the mid-point (called a break barrel), with the barrel serving as a cocking lever. Other systems that are used include side levers, under-barrel levers, and motorized cocking, powered by a rechargeable battery.
Spring guns, especially high-powered ones, have significant recoil resulting from the forward motion of the piston. Although this recoil is less than that of a cartridge firearm, it can make the gun difficult to shoot accurately as the recoil forces are well under way while the pellet is still traveling down the barrel. Most guns seem to respond well to a light, repeatable grip that allows the gun to vibrate the same way from shot to shot. Spring gun recoil also has a sharp forward component, caused by the piston as it hits the forward end of the chamber when the spring behind it reaches full expansion. This sudden forward acceleration helps to counteract the recoil, since the recoil and "forward recoil" forces happen within milliseconds of each other, but it is infamous for the loosening or breaking of lenses and reticles found in low- and medium-pricedtelescopic sights. All mounted telescopic sights for air guns should be rated as such.
Spring guns can also suffer from spring vibrations that reduce accuracy. These vibrations can be controlled by adding features like close-fitting spring guides or by aftermarket tuning done by "air-gunsmiths" who specialize in air gun modifications. A common modification is the addition of viscous silicone grease to the spring, which both lubricates it and dampens vibration.
The better quality spring air guns can have very long service lives, being simple to maintain and repair. Because they deliver the same energy on each shot, their trajectory is consistent. Most Olympic air gun matches through the 1970s and into the 1980s were shot with spring-piston guns, often of the opposing-piston recoil-eliminating type. Beginning in the 1980s, guns powered by compressed, liquefied carbon dioxide began to dominate competition. Today, the guns used at the highest levels of competition are powered by compressed air stored at very high pressures of 2000 to 3000 lb/in² (14 to 21 MPa).[citation needed]
Gas Spring
Some makes of air rifle (e.g. Weihrauch, Theoben) incorporate a gas spring in some models instead of a mechanical spring. Pressurized air or nitrogen is held in a special chamber built into the piston, and this air is further pressurized when the gun is cocked. It is, in effect, a gas spring commonly referred to as a "gas ram" or "gas strut". Gas spring units require higher precision to build, since they require a low friction sliding seal that can withstand the high pressures when cocked. Gas spring units are available as an upgrade for some popular models such as the Weirauch HW80, the Crosman 766C American Classic air rifle and the Arowsmith 876 Magnum rifle. The advantages of the gas spring include the facility to keep the rifle cocked and ready to fire for long periods of time without harming the mechanism. Also, since there is no spring (and therefore a reduction in moving mass during firing) there is less (although some say slightly sharper), recoil. There is also an elimination of the associated problems of long-term spring fatigue and a faster "lock time" (the time between pulling the trigger and the pellet being discharged). The improvement in lock time makes for better accuracy since there is less time for the gun to move off target. Finally, gas springs are practically maintenance free and last longer than conventional metal springs (research test ch1208), however they are an order of magnitude more expensive when they do need replacement.
Pneumatic
Pneumatic air guns utilize pre-compressed air as the source of energy to propel the projectile. Single-stroke and multi-stroke guns utilize an on board pump to pressurize the air in their reservoir, Pre-charged Pneumatic guns' reservoirs are filled using either a high-pressure hand pump (often capable of attaining pressures of 30 MPa) or by decanting the necessary volume/pressure of air from a diving cylinder. Because of this design, having no significant movement of heavy mechanical parts during the firing cycle, the recoil produced is only the "true" recoil, equivalent to the equal and opposite reaction to the pellet and air volume's acceleration up the bore.
Multi-stroke
Multi-Stroke pneumatic air guns require 2-10 pumps of an on-board lever to store compressed air within the air gun. Variable power can be achieved through this process, as the user can adapt the power level for long, or short-range shooting. The design of higher quality and match-grade multi-stroke air rifles can propel a pellet to speeds in excess of 1,000 feet per second (300 m/s)[citation needed].
For beginners and intermediates, multi-stroke air rifles have been a cost-effective choice as they are generally the cheapest form of air gun available. Several manufacturers make multi-stroke air guns including, to name a few, Sheridan, Benjamin, Daisy, and Crosman. Modified multi-pump guns, with stronger pump linkages and improved valves, can produce muzzle energies in excess of 30 foot-pounds force(41 J)[citation needed].
Single-stroke
As the name implies, one motion of the cocking lever is all that is needed to compress the air for propulsion. The single-pump system is usually found in target rifles and pistols, where the higher muzzle energy of a multi-stroke pumping system is not required. Single-stroke pneumatic rifles dominated the national and international ISSF 10 metere air rifle shooting events from the 1970s up to the 1990s.
[edit]Pre-charged pneumatic (PCP)
Pre-charged pneumatic (PCP) air guns are usually filled by decanting from an air reservoir, such as a diving cylinderor by charging directly with a hand pump. Because of the need for cylinders or charging systems, PCP guns have higher initial costs but very low operating costs compared to CO2 guns. These guns are often used for hunting purposes in countries with restrictive firearms laws. A distinction is sometimes made between true PCP guns and HPA guns. The distinction being that true PCP guns are an integrated high pressure design while an HPA application is an adaptation of a high pressure, regulated air supply to function with components not designed for high pressure - e.g. CO2 guns. The RWS/Hammerli 850 is a CO2 designed gun which is often adapted to HPA.
PCP guns have very low recoil and can fire as many as 500 shots per charge. The ready supply of air, has allowed the development of semi-automatic PCP air guns. PCP guns are very popular in the UK and Europe because of their accuracy and ease of shooting. They are widely utilized in ISSF 10 meter air pistol and rifle shooting events and the sport of Field Target shooting,[5] and fitted withtelescopic sights.
Earlier hand pumps for charging carried with them problems of fatigue (both human and mechanical), temperature warping, and condensation. None of those is beneficial to good shooting or the longevity of the rifle. Modern hand pumps have built-in air filtration systems and have overcome many of these problems. Using scuba-quality air decanted from a scuba cylinder also provides a clean, dry, high-pressure air supply that is consistent and available at low cost.
During the discharge cycle, the hammer of the rifle is released by the sear to strike the valve. The hammer may move rearwards or forwards, unlike firearms where the hammer almost always moves forward. Prior to being struck by the hammer, the valve is held closed by a spring and the pressure of the air in the air gun's tank. The pressure of the spring is constant, and the pressure of the air changes with each successive shot. As a result, when the tank pressure is at its peak, the valve permits passage of less total volume of air than when the tank pressure has been reduced by a series of shots. This results in a somewhat greater consistency of velocity from shot to shot than would otherwise be expected, and accuracy with a rifle is mainly dependent on consistency.
Better PCP rifles and pistols are often regulated, i.e. the valve operates within a secondary chamber within which the air pressure is kept constant for a set number of shots, rather than directly within the main reservoir. The pressure within this secondary chamber is maintained at a lower pressure than the pressure in the main reservoir by means of a regulator. Thus shot to shot consistency is far greater than in an unregulated rifle, at least as long as the pressure in the main reservoir is higher or equal to the regulated pressure in the secondary chamber. Beyond this point, the rifle or pistol will operate as any unregulated gun, and velocities drop rapidly.
CO2
Most CO2 guns (e.g. Beretta Elite II) use a disposable cylinder, a powerlet, that is purchased often pre-filled with 12 grams of liquefied carbon dioxide, although some, usually more expensive models, use larger refillable CO2reservoirs like those typically used with paintball markers.
Carbon dioxide-powered guns have two significant advantages over pre-charged pneumatic air guns: (1.) A simpler system for compact storage of energy—a small volume of liquid converts to a large volume of pressurized gas. (2.) No pressure regulator. Within a temperature range tolerable to humans there is little need to regulate the inherently suitable pressure for low-to-moderate-power air guns. The vapor pressure is dependent only on temperature, not tank size, as long as some liquid CO2 remains in the reservoir.
These two advantages allow CO2 guns to be constructed more simply than guns using a pressurized air reservoir. Some CO2-powered guns have detachable or fixed reservoirs that are loaded with pressurized gas from a larger cylinder. Most CO2 powered guns use the standard 12 gram Powerlet disposable cylinder invented by Crosman. Recently, the same company introduced a new 88 gram disposable AirSource cylinder that is used in some of their guns.
On the other hand, liquefied CO2 must be purchased, which introduces an element of cost that does not factor with a PCP gun/hand pump combination using "free" air, or is at least considerably lower when refilling from a diver's tank.
Furthermore, the pressure of gaseous CO2 at ordinary ambient temperatures is only around 850–1000 psi (6 to 7 MPa), which is only a third of the safe working pressure of a typical full PCP reservoir (20 MPa or 2900 psi or more). The effect of this is that generally speaking CO2 guns are lower powered and less efficient than PCP guns, which is why CO2 guns are usually pistols or semi-target type rifles, with few guns (none of commercial note) reaching even the 12 ft (3.7 m)·lbf (16.2 joules) licence-free energy limit for air rifles imposed in the UK.
CO2 guns, like compressed air guns, offer power for repeated shots in a compact package without the need for complex cocking 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 CO2 power. These guns are popular for training, as the guns and ammunition are inexpensive, 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 CO2 powered guns are relatively inexpensive, although there are still a few precision target guns on the market that use CO2.
The CO2 system has been used in experimental non-lethal law enforcement weapons, where high power delivery systems launch rubber batons or bean bags out of a gas-powered launcher, much like a non-lethal shotgun system (but at lower velocities, thus being safer).
refer:wikipedia.org
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