Senin, 26 September 2011

Air Gun Overview

History

Air guns represent the oldest pneumatic technology. The oldest existing mechanical air gun, a bellows air gun dating back to about 1580, is in the Livrustkammaren Museum in Stockholm. This is the timeline most historians peg as the beginning of the modern air gun.
In the 17th century, air guns, in calibers .30 - .51, were used to hunt big game deer and wild boar. These air rifles were charged using a pump to fill an air reservoir and gave velocities from 650-1,000 feet per second. They were also used in warfare; the most famous example is the Girandoni Air Rifle.
At that time, they had compelling advantages over the primitive firearms of the day. For example, air guns could be fired in wet weather and rain (unlike matchlock muskets) and with greater rapidity than muzzle-loading guns. Moreover, they were quieter than a firearm of similar caliber, had no muzzle flash, and were completely smokeless, thus not disclosing the shooter's position. Black powder muskets of the 18th and 19th century produced huge volumes of dense smoke when fired, a disadvantage compared to air rifles.
Although some enthusiasts talk about air guns posing a serious alternative to powder weapons, that was never proved to be the case, as valve leaks and bursting reservoirs were known problems. Air guns also were delicate and crude, and peasant-soldiers, many of whom had never seen any mechanical tools more complex than horse-drawn carriages, could not have operated or maintained them properly. Later improvements in valve designs and reservoir strength either came too late or were too complex for the few air gunsmiths of the day.
But in the hands of skilled soldiers, they gave the military a distinct advantage. France, Austria and other nations had special sniper detachments using air rifles. The Austrian 1780 model was named Windbüchse (literally "wind rifle" in German). The gun was developed in 1778 or 1779 [1] by the Tyrolese watchmaker, mechanic and gunsmith Bartholomäus Girandoni (1744-1799) and is sometimes referred to as the Girandoni Air Rifle or Girandoni air gun in literature (the name is also spelled "Girandony," "Giradoni"[2] or "Girardoni".[3]) The Windbüchse was about 4 ft (1.2 m) long and weighed 10 pounds (4.5 kg), which was about the same size and mass as a conventional musket. The air reservoir was a removable, club-shaped butt. The Windbüchse carried twenty-two .51 in (13 mm) lead balls in a tubular magazine. A skilled shooter could fire off one magazine in about thirty seconds, which was a fearsome rate of fire compared to a muzzle loader. A shot from this air gun could penetrate a one-inch wooden board at a hundred paces, an effect roughly equal to that of a modern 9 mm or .45 ACP caliber pistol.
Air gun developed by the Japanese inventor Kunitomo, circa 1820-1830.
Kunitomo air gun trigger mechanism.
Around 1820, the Japanese inventor Kunitomo Ikkansai developed various manufacturing methods for guns, and also created an air gun based on the study of Western knowledge ("rangaku") acquired from the Dutch in Dejima.
Air guns appear throughout other periods of history. The celebrated expedition headed by Lewis and Clark (1804) carried a reservoir air gun, later believed to be the Girandoni Military Repeating Air rifle in Dr Robert Beeman's Collection.[citation needed] It held 22 round balls in a tubular magazine mounted on the side of the barrel. The butt stock served as the air reservoir and had a working pressure of 800 PSI. The rifle was said to be capable of 22 aimed shots in one minute. However, that air rifle is measured to have a rifled bore of .452" and a groove diameter 0.462".
During the 1890s, air rifles were used in Birmingham, England, for competitive target shooting. Matches were held in public houses, which sponsored shooting teams. Prizes, such as a leg of mutton for the winning team, were paid for by the losing team. The sport became so popular that just after the turn of the 19th century,[when?] a National Air Rifle Association was created. During this time over 4,000 air rifle clubs and associations existed across Britain, many of them in Birmingham. During this time, the air gun was associated with poaching because it could deliver a shot without a significant report.
Today's modern air guns are typically low-powered because of safety concerns and legal restrictions; however, high-powered designs are still used for hunting. These air rifles can propel a pellet beyond 1100 ft/s (330 m/s), approximately the speed of sound, and produce a noise similar to a .22 caliber rimfire rifle. Using lead pellets, some current spring powered .177 pellet guns can break the sound barrier. Most low-powered air guns can be safely fired in a backyard or garden, and even indoors, with a proper backstop.
In some countries, air guns are still classified as firearms, and as such it may be illegal to discharge them in residential areas. Air guns can be highly accurate and are used in target shooting events at the Olympic Games, governed by the International Shooting Sport Federation (ISSF).

Use

Air guns are used for hunting, pest control, recreational shooting (commonly known as plinking), airsoft fights,(in which players fire airsoft guns at each other). and competitive sports, such as the Olympic 10 m Air Rifle and 10 m Air Pistol events. Field Target (FT) is a competitive form of target shooting in which the targets are knock-down metal silhouettes of animals, with a 'kill zone' cut out of the steel plate. Hunter Field Target (HFT) is a variation, using identical equipment, but with differing rules. The distances FT and HFT competitions are shot at range between 10 metres and 50 metres, with varying sizes of 'reducers' being used to increase or decrease the size of the kill zone. In the UK, competition power limits are set at the legal maximum for an unlicensed air rifle, i.e. 12 ft·lbf.

Legal issues

The legal definition of an air gun differs from country to country. There may be minimum ages for possession and sales of both air guns and ammunition may be restricted. Some areas may require permits and background checks similar to those required for firearms possession.

Air gun power sources

There are different methods of powering an air gun. These methods can be broadly divided into 3 groups: spring-piston, pneumatic, and CO2. These methods are used in both air rifles and air pistols.[4]

Spring-piston

Single shot, break barrel, spring-piston air rifle
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-priced telescopic 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

 
The Walther LGR single-stroke pneumatic match air rifle.
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.

Pre-charged pneumatic (PCP)

 
An example of a pre-charged pneumatic air pistol, as used in 10 metre air pistol ISSF shooting events.
Pre-charged pneumatic (PCP) air guns are usually filled by decanting from an air reservoir, such as a diving cylinder or 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 with telescopic 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

 
BB Pistol with CO2 cartridges and BBs. This pistol can shoot at up to 480 ft/s (150 m/s). It is a Daisy 15xt air pistol.
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 CO2 reservoirs 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).

Safety

For safety, CO2 containers must be kept at temperatures below 120 °F (49 °C) ; at temperatures above this level, the pressure begins to increase very rapidly, and can cause the container to fail. CO2 containers with diameters at or above two inches (50 mm) have a pressure release "rupture" mechanism to release the contents over a certain pressure level and avoid explosion because of high temperature. These disks are generally calibrated to a minimum pressure corresponding to the 120 °F (49 °C) level at 100% of the rated CO2 capacity. Elevated temperatures, even those below the critical temperature, can cause increased leaking through seals.

Operating considerations

  • Re-filling Forcing more carbon dioxide gas into a reservoir of liquid and gas CO2 while maintaining a constant temperature would not raise the pressure but merely convert the additional gas into liquid. By chilling the vessel to be filled, the lower vapor pressure will pull CO2 from the source container. While the pressure in the reservoir is generally dependent only on the temperature, if the bottle is too full, that changes. The expansion of the liquid CO2 will take up all the space in the bottle, preventing evaporation. At this point, the pressure increase with temperature becomes dangerously high.
  • Cooling Each time the gun is fired there is some sublimation of solid to gas which is an endothermic process in which the pressure drops until enough ambient heat is absorbed to restore the pressure. When shooting at a rate faster than the cylinder can absorb heat from the environment to counter the cooling of the evaporating liquid, the pressure will drop, and the velocity is likely to drop as well in a non-regulated gun.

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