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=== Spring-piston === [[File:Air rifle collection 2.jpg|thumb|Variety of different types & models of spring piston & gas ram air guns]] A '''spring-piston air gun''' (also known as a '''spring gun''' or simply a "'''springer'''") operates by means of a [[spring (device)|spring]]-loaded [[piston pump]] assembly contained within a compression chamber separate from the [[gun barrel]]. Traditionally, a [[grease (lubricant)|grease]]-lubricated steel [[coil spring]] is used as the powerplant main spring. Before shooting, the user needs to manually cock the gun by flexing a lever connected to the pump assembly, which pulls the pump piston rearwards and compresses the main spring until the rear of the [[piston]] engages the [[sear (firearm)|sear]]. When shooting, the [[trigger (firearms)|trigger]] is pulled and disengages the sear, allowing the main spring to decompress and release its stored [[elastic potential energy]], pushing the piston forward, thereby compressing the air in the pump cylinder. Because the pump outlet (located to the front of the pump) is directly behind the pellet seated in the barrel [[chamber (firearms)|chamber]], once the air pressure has risen enough to overcome the [[static friction]] and/or barrel restriction holding back the pellet, the pellet is propelled forward by an expanding column of pressurized air. All this takes place in a fraction of a second, during which the air undergoes [[adiabatic process#Adiabatic heating and cooling|adiabatic heating]] to several hundred degrees and then cools as the air expands. This can also cause a phenomenon referred as "dieseling", where flammable substances in the compression chamber (e.g., petroleum-based lubricant) can be [[compression ignition|ignited by the compression heat]] like in a [[diesel engine]], and lead to an [[afterburner]] effect with (often unpredictable) additional thrusts. This often results in combustion smoke coming out of the [[muzzle (firearm)|muzzle]] and potential pressure damage to the rubber [[O-ring]] [[seal (mechanical)|seal]]s inside the gun. Dieseling can be made to occur ''intentionally'' to increase power, by coating the pellet with lubricant or [[petroleum jelly]], although this may result in damage to the breech seal. Most spring-piston guns are [[single-shot]] [[breechloader]]s by nature, but multiple-shot [[repeating rifle|repeater]]s with [[magazine (firearms)|magazine]] feeders have been introduced in recent years by manufacturers such as [[Gamo (airgun manufacturer)|Gamo]], [[Umarex]] and Hatsan. [[File:Air Pistol.jpg|thumb|Spring Piston Air Pistols]] Spring-piston guns, especially the high-powered "magnum" guns, are able to achieve muzzle velocities exceeding the [[speed of sound]]. The effort required for the cocking stroke is proportional to the designed power of the gun, with higher muzzle velocities requiring a stiffer spring and hence a greater cocking effort. Spring-piston guns have a practical upper limit of {{convert|1250|ft/s|m/s|abbr=on}} for [[.177 caliber|.177 cal]] (4.5 mm) pellets, as higher velocities cause unstable pellet flight and loss of accuracy. This is due to the extreme [[Aeroelasticity#Buffeting|buffeting]] caused when the pellet reaches and surpasses [[transonic]] speed, then slows back down and goes through [[sound barrier]] again, which is more than enough to destabilize the pellet's flight. 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 stability to be disrupted. Drag increases disproportionately as pellet velocity increases, so it is generally better to increase pellet weight to keep velocities subsonic in high-powered guns. Many shooters find that velocities in the {{convert|800|-|900|ft/s|m/s|abbr=on}} range offer an ideal balance between power and accuracy. Spring guns are typically cocked by one of the following mechanisms: * Break-barrel β like a [[break action]] firearm, the barrel is hinged at the junction with the [[receiver (firearms)|receiver]] and is flexed downwards to expose the [[breech (firearms)|breech]] and serves as the cocking lever * Fixed-barrel β the barrel is fixed to the receiver, and the cocking is done with a separate cocking lever ** Underlever β the cocking lever is located underneath the barrel or the receiver and is flexed downwards during cocking ** Sidelever β the cocking lever is located to the side (usually the right side) of the receiver and is flexed sideways during cocking ** Overlever β the cocking lever is located above the barrel and is flexed upwards during cocking, seen in some air pistols * Motorized cocking powered by a rechargeable battery (rare) Spring-piston guns, especially high-powered ("magnum") models, do still [[recoil]] as a result of the mainspring pushing the piston forward. Although the recoil is less than that of some [[Cartridge (firearms)|cartridge]] firearms, it can make the gun difficult to shoot accurately as the spring recoil is in effect while the pellet is still within the barrel. Spring gun recoil also has a sharp forward component, caused by the piston hitting the front end of the pump chamber when the spring has fully decompressed. These rapid double-jerking movements are known to damage [[telescopic sight|scopes]] not rated for spring gun usage. In addition, the spring often has unpredictable collateral transverse vibrations as well as [[torque|torquing]], both of which can cause accuracy to suffer. These vibrations can be controlled by adding features like close-fitting spring guides or by aftermarket tuning done by gunsmiths who specialize in air gun modifications, a common one being the addition of high viscosity tenacious grease to the spring, which lubricates and serves to dampen vibration. The better quality spring guns can have very long service lives, being simple to maintain and repair. Because they deliver the same mechanical energy output on each shot, external ballistics are quite consistent.{{citation needed|date=November 2019}} Most [[Olympic Games|Olympic]] [[ISSF shooting events|air gun competitions]] 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 the competition. Today, the guns used at the highest levels of competition are powered by [[compressed air]]. ==== Gas spring ==== Some newer generation air guns incorporate a [[gas spring]] (commonly referred to as a ''gas piston'', ''gas ram'', ''gas strut'' or ''nitro piston'') instead of a mechanical spring. The spring itself is essentially a stand-alone enclosed [[piston pump]] without outlets and with pressurized air or [[inert gas]] (such as [[nitrogen]]) held tightly sealed within the [[cylinder (engine)|cylinder]]. When the gun is cocked, the gas inside the cylinder gets further compressed by the piston, stores [[potential energy]] and acts as a [[pneumatic]] [[accumulator (energy)|accumulator]]. Gas spring units require higher precision to manufacture since they require a low-friction sliding seal that can withstand the high pressures within when cocked. The advantages of the gas spring include the ability to keep the gun cocked and ready to fire for extended periods of time without long-term spring [[fatigue (material)|fatigue]], no twisting [[torque]] (caused by coil spring expanding) exerted onto the gun, smoother recoil pattern and faster "lock time" (the time between pulling the trigger and the pellet being discharged), which all result in more consistent accuracy. Gas springs perform more reliably in cold climates than coil springs, because metallic coil springs need to be coated with [[grease (lubricant)|lubricating grease]] which often thickens in low temperatures causing the gun to "freeze up". Gas springs also have less lateral and longitudinal vibrations than coil springs, hence are usually less "hold-sensitive", making it easier to achieve consistent [[shot grouping]]s.
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