How Airguns Are Powered

How Airguns Are Powered: 

How airguns are powered varies greatly. There are different methods of powering an air gun and they can be broadly divided into 4 groups: spring/gas-piston, pneumatic, PCP and CO₂ . These methods are used in both air rifles and air pistols.

The Spring-Piston Power Plant

Spring/Gas-Piston

Spring-piston airguns 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 effort required for the cocking stroke is usually related to the power of the gun, with higher muzzle velocities requiring greater effort.

Spring-piston airguns 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 the rear of the piston engages the sear. The act of 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 1250 ft/s (380 m/s) for .177 cal (4.5 mm) pellets. Higher velocities cause unstable pellet flight and loss of accuracy. This is due to the extreme buffeting caused when the pellet reaches and passes transonic speed, then slows back down and goes through it again. This is more than enough to destabilize it. Shortly after leaving the barrel, the supersonic pellet falls back below the speed of sound and the shock wave overtakes the pellet, causing its flight to be disrupted. 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. Many shooters have found that velocities in the 800–900 ft/s (240–270 m/s) range offer an ideal balance between power and pellet stability.

Most spring piston airguns 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 midpoint (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 piston guns, especially high-powered ones, recoil as a result of the forward motion of the piston. Although the recoil is less than that of some cartridge firearms, it can make the gun difficult to shoot accurately as the recoil forces are in effect whilst the pellet is still traveling down the barrel. Spring gun recoil has a sharp forward movement too, caused by the piston hitting the forward end of the chamber when the spring has fully expanded. These two reactions are known to damage telescopic sights not rated for spring gun use.

Spring airguns 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.

How Airguns Are Powered: The Gas Piston

Some makes of air rifle incorporate a gas spring instead of a mechanical spring. Pressurized air or nitrogen is held in a 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. The advantages of the gas spring include the facility to keep the rifle cocked and ready to fire for long periods of time without long-term spring fatigue, lower recoil and faster “lock time” (the time between pulling the trigger and the pellet being discharged). The improvement in lock time results in better accuracy.

The Pneumatic Pump

Pneumatic air guns utilize compressed air as the source of energy to propel the projectile. Single-stroke and multi-pump guns utilize an on board pump to pressurize the air in a reservoir. Pre-charged pneumatic guns’ reservoirs are filled using either a high-pressure hand pump or by decanting air from a diving cylinder. This design, having no significant movement of heavy mechanical parts during the firing cycle, produces lower recoil.

Multi-stroke/pump

Multi-pump pneumatic air guns require the pumping 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.

Single-stroke

A single motion of the cocking lever is all that is required to compress the air. 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 10m air rifle shooting events from the 1970s up to the 1990s.

How Airguns Are Powered: The Pre-Charged Pneumatic (PCP)

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 much lower operating costs when compared to CO₂ guns. 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 and fully automaticair guns. PCP guns are very popular in the United Kingdom and in Europe due to their accuracy and ease of use. They are widely utilized in ISSF 10m air pistol and rifle shooting events the sport of Field Target shooting, and are usually fitted with telescopic sights.

Early hand pump designs encountered problems of fatigue (both human and mechanical), temperature warping, and condensation. None of which are beneficial to accurate shooting or air gun longevity. 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 provides consistently clean, dry, high-pressure air.

How Airguns are Powered with Unregulated action:

During the typical PCP’s discharge cycle, the hammer of the rifle is released by the sear to strike the bash valve. The hammer may move rearwards or forwards, unlike firearms where the hammer almost always moves forward. The valve is held closed by a spring and the pressure of the air in the reservoir. The pressure of the spring is constant, and the pressure of the air decreases with each successive shot. As a result, when the reservoir pressure is at its peak, the valve opens less fully and closes faster than when the reservoir pressure is lower, resulting in a similar total volume of air flowing past the valve with each shot. This results in a degree of self-regulation that gives a greater consistency of velocity from shot to shot. A well-designed PCP will display good shot to shot consistency as the air reservoir is depleted.

How Airguns are Powered with Regulated action:

Other PCP rifles and pistols are regulated, i.e. the firing valve operates within a secondary chamber separated from the main air reservoir by the regulator body. The regulator maintains the pressure within this secondary chamber at a set pressure (lower than the main reservoir’s) until the main reservoir’s pressure drops to the point where it can no longer do so. As a result, shot to shot consistency is maintained for longer than in an unregulated rifle.

How Airguns Are Powered: CO2 Airguns

Airgun Pistol Powered By Co2 and disposable cylinders

 

Most CO₂ airguns use a disposable cylinder, a powerlet, that is purchased often pre-filled with 12 grams of pressurized 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. So, as long as the CO₂ cylinder is not allowed to cool significantly (through firing), the pressure will not decrease by a significant degree. However, at higher temperatures (e.g. on a hot summer day), this approximation does not hold true.

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 standardized by Crosman.

CO₂ guns, like compressed airguns, 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.

How Airguns Are Powered: 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 rapid evaporation of liquid 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.

Learn More About Airguns:

WHAT IS AN AIRGUN

AIRGUN HISTORY

WHY BUY AN AIRGUN

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