Spring-piston air guns are able to achieve muzzle velocities near 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 cocking effort. For example Gamo's Hunter Extreme at 1,600 ft/s (490 m/s) has a cocking effort of 58 lb (26 kg) while those shooting 1,000 ft/s (300 m/s) or 1,250 ft/s (380 m/s) usually require only 33 lb (15 kg).
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 during compression, and then cools as the air expands once more.
Modern air gun lubricants are generally a compounded mix of ingredients, such as silicone paste and
molybdenum disulfide. These compounds are designed to not burn at the temperatures reached in airgun compression chambers, however it seems that any form of lubricant will burn to some extent at these elevated temperatures. Before the availability of synthetic lubricants, when purely petroleum based products were used, some writers claimed that upwards of 30% of the energy of the shot may have come from the burning of some of the lubricant although this has been debated by others. The Cardews experimented by firing a spring-piston air rifle within a nitrogen-only enclosure, thus eliminating oxygen and preventing any form of combustion. The resultant shots fired displayed very noticeably reduced power when compared to the same setup fired both before and after the experiment outside of the enclosure, i.e. within the Earth's atmosphere. The limited burning of very small quantities of lubricant is not to be confused with "
dieseling" which occurs when volatile fractions and vapours of petroleum-based lubricants burn violently, generally the result of excessive lubricant finding its way (or deliberately placed) ahead of the piston. This can and often does severely damage the spring and piston seals, and in extreme cases cause the cylinder to bulge, especially in modern, more highly stressed guns. Dieselling is noticeable also because the gun emits a very loud report, often comparable to that produced by a
.22 Short rimfire cartridge, some muzzle flash, and smoke.
Spring-piston guns seem to 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] However, the longer lighter pellets can achieve good accuracy and surprising high speed (the Gamo hunter extreme can reach 1,600 ft/s (490 m/s) with their special pellets). 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. 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.[
citation needed]
Most spring piston guns are single-shot breech-loaders by nature (somewhat like a single or double barreled
shotgun) but multiple-shot guns have been increasingly 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 a 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 backward recoil, since the backward and forward recoil forces happen within milliseconds of each other, but it is infamous for knocking around and loosening or breaking the lenses and reticles found in low and medium priced
telescopic sights, even those which are designed to withstand the (backward-only) recoil from high-powered firearms. On any but the lowest power spring guns, any mounted telescope should be air gun rated.
Spring guns can also suffer from spring vibrations that can upset accuracy. These vibrations can be controlled by adding features designed into the gun, like close-fitting spring guides, or by aftermarket tuning done by 'airgunsmiths' who specialize in air gun modifications. A common modification is the addition of viscous silicone grease to the spring, which both lubricates it and damps out vibration.[
citation needed]
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, the trajectory is extremely consistent. This resulted in most
Olympic air gun matches through the 1970s and into the 1980s being shot with spring-piston guns, albeit 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).
The Chinese army uses spring-piston small arms to train more economically. These military-issue Chinese spring-piston air-guns are often available by mail-order, but the buyer should note that quality control on these guns tends to be somewhat variable. Similarly, the Romanian army formerly used spring-piston, single shot, 4.5 mm training rifles to train entry level recruits, prior to switching them to .22 LR training rifles.
Some makes of air rifle (e.g. Weihrauch, Theoben) incorporate a gas spring 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 rifles 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 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.
