Technical Information About Firearms

What is an Air Gun

image

Air guns represent the oldest pneumatic technology dating back to 1580, where historians recognise this as the beginning of the modern air gun. In the 17th Century air gun calibers of .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 of 650 to 1,000 feet per second (200-300m/s).

During this time they had compelling advantages over the primitive firearms of the day, due to air guns being able to fire in wet weather and rain unlike matchlock muskets and fired with greater speed than muzzle-loading guns. Moreover operationally they were quieter and did not reveal the shooter’s position as there was no muzzle flash and were smokeless unlike the Black Powder muskets of the 18th and 19th centuries.

There was an Austrian 1780 model named the Windbuchse (translated as “wind rifle” in German), this gun was developed in 1778 by the Tyrolean watchmaker, mechanic and gunsmith Bartholomaus Girandoni (1744-1799) and is sometimes referred to as the Girandoni Air Rifle or Girandoni air gun in literature. The Windbuchse was about 4ft (1.2m) long and weighed 4.5kg about the same size and mass as a conventional musket of the same period. The air reservoir was a removable, club-shaped buttoned carried twenty-two .51 caliber (13mm) lead balls in a tubular magazine.

In the hands of a skilled soldier they gave the military a distinct advantage, a skilled shooter could fire off one magazine in about thirty seconds and a shot from a Windbuchse could penetrate an inch thick wooden board at a hundred paces, an effect roughly equal to that of a modern 9mm or .45 ACP caliber pistol. Fast forward to the 1890′s air rifles were used in Birmingham in the UK for competitive target shooting, matches were held in public houses, which sponsored the teams. Prizes such as a leg of mutton for the winning team was paid for by the losing team. The sport became so popular that in 1899, the National Smallbore Rifle Association was created. During this time over 4,000 air rifle clubs and associations existed across the UK and many of them were in Birmingham.

Air Gun Power Sources

The different methods of powering an air gun can broadly be divided into 3 groups, spring-piston, pneumatic and CO2 these methods of power are the same for air rifles and pistols.

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 effort required for 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 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 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 shock wave generated as the supersonic pellet contacts the air. Shortly after leaving the barrel, the pellet falls below the speed of sound and the shock wave overtakes the pellet, causing it to tumble. 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 (unless the gun is silenced). 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 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 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 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.

Gas Spring

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.

Pneumatic

Pneumatic air guns utilize 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 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

Multi-stroke 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 10 metre air rifle shooting eventsfrom the 1970s up to the 1990s.

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 automatic[7][8] air guns. PCP guns are very popular in the UK and Europe because of their accuracy and ease of use. They are widely utilized in ISSF 10 metre air pistol and rifle shooting events, the sport of Field Target shooting,[9] 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.

CO2

Most CO2 guns 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 pressurised 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 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 specialised 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 of CO2

For safety, CO2 containers must be kept at temperatures below 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 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 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 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.

Ammunition

Pellet

The most popular ammunition used in rifled air guns is the lead diabolo pellet. This waisted projectile is hollowed at the base and available in a variety of head styles. The diabolo pellet is designed to be drag stabilized, though is not as stable as some other shapes in the transonic region (272–408 m/s ~ 893–1340 ft/s). Pellets are also manufactured from tin, or a combination of materials such as steel-tipped plastic.

Most air guns are .177 (4.5 mm) or .22 (5.5 mm / 5.6 mm), and are designed for target practice, small game hunting and field target shooting.

  
image

Rim Fire Explained

Dunfermline Small Bore Rifle Club uses what’s known as Rim Fire Ammunition which is a type of firearm cartridge, thus known because instead of the firing pin in the rifle striking the centre of the base of the cartridge to ignite it (as in a centre fire cartridge), the firing pin strikes the base’s rim.

When you examine a rimfire cartridge you will notice that it is extended and widened at one end, this is made from brass and contains the priming compound, while the cartridge case itself contains the propellant powder and the bullet.

Once the rim of the cartridge has been struck and the bullet discharged, the cartridge cannot be re-loaded, because the head of the cartridge has been de-formed by the impact of the firing pin.

Recycling

Once fired the cartridge has been used it is discarded in the recycling container in the firing range, these must be collected after each session in the range to avoid creating trip hazards and for good housekeeping.

While many other different cartridge priming methods have been tried since the 19th century, only rimfire technology and centerfire technology survive today in significant use.