Spring-powered airsoft guns are single-shot devices that use elastic potential energy (EPE) stored in a spring to compress air to launch an airsoft pellet down the barrel of the gun. The user must cock a spring gun prior to each shot. This is typically achieved by pulling back the slide (pistols), bolt (rifles), or the grip on a shotgun, which in turn compresses the spring and makes the gun ready to fire. Because of this, these guns are by definition incapable of automatic or semi-automatic fire. Spring-powered airsoft guns are generally not as powerful as gas and electric models, although some spring shotguns and sniper rifles can be very powerful and shoot at velocities of 400–700 FPS. These are not hard to find and are generally inexpensive, and don't usually last long because of the tension created by a powerful spring.
While most electric guns also use springs for propulsion of the airsoft pellet, they are not considered to be in the same category as the single-shot spring-powered guns. Low-end spring guns tend to be much cheaper than their electric-powered equivalents due to their simplicity and lack of electrical components (spring assembly, electric motor, battery, and battery charger) and thus are widely available. These guns are less suited for competition because they are at a disadvantage against automatic guns in close combat and do not provide enough accuracy and power for long-range use. There are some exceptions, however, as higher-end spring-powered airsoft rifles can be quite expensive; these guns are typically suited for "marksman" applications in airsoft matches and provide competitive muzzle velocities. Additionally, pump shotguns are sometimes used, especially in CQB (Close Quarters Battle). In colder weather, spring pistols are more reliable than gas-powered pistols and even the batteries on AEPs (Automatic Electric Pistols) both of which can be adversely affected by extreme cold.
This represents one of the major advantages of spring-powered airsoft gun, as it can be fired in any situation, without reliance on an external source of power, such as batteries or gas. The lack of reliance on external power sources causes some players to favor spring powered guns. Spring guns are also less susceptible to the effects of water, where a battery-powered gun could malfunction when wet.
Spring-powered weapons are often cheaper than electric or gas powered weapons. They are also more readily available in most department stores. Because of their price and availability, spring guns tend to act as "training guns" to bring new players to airsoft games and are considered the primary weapon of "backyard skirmishes". Almost all airsoft players at some point owned a spring weapon, whether for its actual use in the sport or for the replica value since some airsoft weapons are only available as spring versions.
Electric guns, in their varying forms, use an electrically powered gearbox to compress a spring, which, when released, propels the pellet out of the barrel.
Automatic electric gunsEdit
An illustration of the working of a Version 2 gearboxJing Gong Steyr AUG AEGElectric-powered airsoft guns typically use a rechargeable battery to drive an electric motor, which cycles an internal piston/spring assembly in order to launch pellets. Automatic, 3 round burst, and semi-automatic operation is possible which gives these guns the popular name "automatic electric guns" or AEGs. These guns often attain muzzle velocities between 150 and 650 ft/s and rates of fire of between 100 and 1500 rounds per minute. They are the most commonly used and widely available type of airsoft gun.
These type of guns were developed in Japan and the Japanese company Tokyo Marui dominates the market. In a Tokyo Marui AEG, the motor drives a series of 3 gears mounted inside a gearbox. The gears then compress a piston assembly against a spring. Once the piston is released, the spring drives it forward through the cylinder to push a pellet into the chamber, through the barrel, and forward from the muzzle. Many manufacturers have now more or less replicated this basic model, adding reinforced parts or minor improvements. These guns are powered primarily by nickel metal hydride (NiMH) with varying voltages and milliampere hours ratings. The most common battery is an 8.4 V large battery (between 2200-5000 mAh.) Also available are "mini" and "stick" batteries, which generally have 900-1600 mAh capacities. Voltages for NiMH batteries range from 7.2 V, all the way up to 12 V. The rule of thumb usually is the higher the mAh, the longer the battery lasts while the higher voltage, the higher Rate of Fire (RoF). Recently, however, Lithium-Polymer, or Li-po, batteries are becoming more popular in the airsoft world. These batteries can last longer and have higher mAh and Volts while at the same time, being small and light. Li-po batteries are usually at 11.1 V or 7.4 V and at varying mAh from 500 mAh to 6500 mAh.
External modifications, such as metal bodies and reinforced plastics that make AEGs look and feel even more realistic, have become very popular. AEG manufacturers such as Classic Army and Tokyo Marui produce replicas that are visually nearly identical to their real counterparts. Tokyo Marui, however, sticks with a durable ABS plastic, whereas Classic Army features full metal bodied guns and stronger furnishings. Most AEGs produced as of late are designed to be as visually realistic as possible.
The three most common AEGs on the field are the AR-15 series (M16 rifle, M4 carbine, etc.; sometimes referred to as the ArmaLite or Colt series), the Heckler & Koch MP5 series, and the AK or Kalashnikov series. Also increasing popular is the Heckler & Koch G36 and more recently, FN P90. Subsequently numerous parts for repairs and modifications are commonly available for these rifles.
Hybrid Airsoft guns are the newest type of airsoft guns on the market. Hybrid airsoft guns are basically standard AEGs with a "little extra reality" built in. The magazine is loaded with shell cases, each containing a single plastic pellet. These shell casings can have a small red cap, the same as those found in any child's toy cap gun placed on the top of them. These guns feature an electrically powered, full blowback system and operate on a "round-per-shell" basis such that for every pellet fired, a shell casing is ejected and the cap is fired providing a realistic sound and smoke effect. Since its debut, the only Hybrid guns seen on the market are cheap non-scaled M4 series and AK series guns, as well as M1 Garand, Kar98, and other rifle models. These guns are the least common type of gun on the market today.
Low powered electric gunsEdit
Dboys selective fire "Boys" Colt Commando electric rifle, approx. 3/4 scale, pictured with .12 gram Crosman ammunition and CybergunCalled low powered electric guns (LPEGs) to distinguish them from the original, more expensive and more powerful AEGs even though their mechanical/electrical design and operation is similar. They are not to be confused with Mini Electrics (described below). Originally they were only of novelty value, often regarded below spring operated guns due to their construction and low velocities. Since there are spring action guns that can notably outperform the true low end LPEGs and can be found at comparable prices, they are generally considered to be better choices.
Medium powered electric gunsEdit
Some companies - like UTG with their popular MP5 and AK-47 models - have improved their quality to such an extent that some models are now considered simply as mid-ranged AEGs that are more affordable but still reasonably effective. Among airsofters, these are commonly called middle powered electric guns (MPEGs). Sometimes, MPEGs are copies or 'clones' of designs by full-price manufacturers like Tokyo Marui. As of late 2008 a small number of MPEG brands such as Echo-1/Jing Gong are considered by many to approach the quality and match the performance of the originals, at less than half the price. "Fully-compatible" MPEGs imitate the Marui or Classic Army originals so precisely that standard upgrade parts will work with them, making it possible to hot-rod an MPEG to well beyond stock out-of-the-box AEG performance.
Electric Blow BacksEdit
Electric Blow Backs, also known as EBBs, are cheap electric guns, mostly modeled after real world pistols, which typically run from four AA batteries, AAA batteries, or rechargeable batteries. EBBs generally have less of a kick versus like a Gas Blow Back to simulate the action of a real pistol. However, a draw back to having the blow back feature is that the battery is quickly depleted.
Electric Blowback can also refer to a feature in some higher end guns which offers more realistic operation. Companies such as G&G now offer guns the Combat Machine M4 which has moving parts linked directly to the main mechanism of the gun, such as the bolt. Echo 1 has recently released a Blow Back MP5SD. These weapons perform identically to similar non-blowback offerings, with the added realism of reciprocating bolts and some recoil. Most incorporate Pneumatic blow back systems but some models are mechanical systems.
Recently, the company UHC, well known for its spring guns, began manufacturing a range of electric guns in miniature size that fire only full automatic. They differ from LPEGs in that they are not replicas of real firearms, being miniaturized version of real firearms, mostly made of black or clear plastic.
They have a small ammo capacity, usually between 50 and 100 rounds, but they have good range and functional hop-up. They have become very popular in recent years, and are now being manufactured by Tokyo Marui, as well as the Taiwanese company HFC. These “minis,” as they are referred to, are not a viable option in games against AEGs since their small ammo capacity, short range and poor far range accuracy leave their wielder at a large disadvantage. Mini electric guns are able to compete with spring pistols at close ranges however, primarily due to their higher rate of fire.
Automatic Electric PistolsEdit
Automatic Electric Pistols, abbreviated AEPs, were first introduced by Tokyo Marui in 2005 with their Glock 18C (followed later by a Beretta 93R model). They were the first handguns to incorporate an electric powered system that is capable of fully-automatic operation.
In cold weather, AEPs are often considered better sidearms than gas powered pistols, because batteries are not as badly affected by frigid weather. Gases like CO2 and green gas are stored in liquid form and require heat in order to vaporize. A gas pistol at 10°F will usually only get one to two usable shots from a full magazine, and even they will be at reduced power because of the lowered pressure of the gas.
Because the gearbox and battery are smaller, the velocity of the pellets is relatively slow for airsoft play, and therefore are only good for close range. The velocity of an AEP is usually between 200 to 280 ft/s (85.3 m/s). However, the advanced hop up units on these new guns tend to compensate for the low power and can produce an effective range comparable to those of an AEG. CYMA has made a clone Glock 18C, which is a lower priced alternative.
An AEP differs from electric blow-backs because the AEP has a fixed slide (in which there is no external movement of the slide during operation), while an EBB attempts to simulate the "blow back" action in the slide experienced in a real pistol or Gas Blow Back (GBB). An AEP, however, has much more power and accuracy.
One of the newer AEP-styled guns is the Marui replica of the Heckler & Koch MP7. It is considerably larger than either of the other guns, and can be upgraded to a much higher power through the use of an external battery, but uses the same system as the AEP, so the classification is ambiguous. It is slightly more powerful than the others and is a suitable choice for CQB (Close Quarter Battle) games due to its small size and decent barrel- to gun-length ratio.
Some semi-automatic pistols can be modified to be automatic pistols. To make them more effective, they often use AA batteries or AAA batteries, and can be replaced with a 9v battery to make their ROF higher; this procedure, though, can often wear-out the airsoft gun, since a motor typically running from 4 AA/AAA batteries will take 6v to run, and a 9v would therefore theoretically put 2.25x as much energy through it since power is proportional to Voltage squared over a fixed resistance.
Due to restrictions on size, either the electric motor or batteries have to occupy space in the hand grip, reducing the available space for a magazine. Because of this no AEP uses a full size magazine found in most gas powered pistols. Many airsoft players find these stick-like plastic magazines too frail and breakable to endure an airsoft skirmish.
In addition, most AEPs are constructed almost entirely of plastic and have a light, toy-like feeling to them.
Gas powered airsoft gunsEdit
A gas blowback Heckler & Koch USP Compact replica, made by KSC.Gas-powered airsoft guns use pressurized gas to propel pellets. These guns are capable of automatic and semi-automatic operation. The most common gases used are "green gas" (which consists of a mixture of propane and a polysiloxane lubricant) and HFC-134a. Less commonly used gases include "red gas" (which is actually HCFC-22), CO2 and nitrogen/high pressure air. It is unlawful to use HCFC-22 as a propellant in the US. HCFC-22 is a Class II ozone depleting substance and its use as a aerosol propellant has been banned since January 1994 under section 610(d) of Clean Air Act. 
Red gas is usually avoided unless the airsoft gun has undergone modification, as its relatively high critical pressure can cause damage to the airsoft gun, such as breakage of the slide or bolt. CO2, nitrogen, and high pressure air are less common because they need to be stored at higher pressures than "green gas" or HFC-134a.
The first ever gas powered airsoft guns were commonly referred to as 'classic' guns, owing to their age. These guns were most commonly powered by liquid propellants such as R-12 (Which was marketed by the Japanese as FLON-12 or DuPont tradename Freon 12)CFC-12 feed system with a majority of the configurations containing two tanks, one containing the CFC-12 and one used as an expansion tank, and the gun itself. CFC-12 was a commonly used refrigerant for car air conditioning and refrigerators.
It is considered a highly potent ozone depleting substance and listed as a Class I Ozone Depleting Substance by the US EPA. Its use as a general purpose aerosol propellant has been banned by the US EPA since March 1978 under 43 FR 11301 for use in aerosol use with a very few exceptions.  Its use is also banned in many countries under United Nations treaties. On Dec 31, 2008, the use of CFCs for medical inhalers were banned.
The use of Later users modified these old guns to be powered by regulated CO2 canisters or nitrogen/high pressure air bottles to increase power and consistency. However, these guns have largely been superseded by the newer and more versatile AEGs, or automatic electric guns. One of the reasons for this is because the most commonly available propellant, R-12, is costly. Additionally, at high flow rates, liquid propellants tend to cool down, eventually freezing. As cooldown progresses, the rate of fire gradually decreases until the gun ceases operation. The user would then be forced to wait for the propellant to warm up again. CO2 is not affected as badly by this tendency, and nitrogen/high pressure air is immune to it. Furthermore, if liquid propellant is introduced into the gun's mechanism, rubber parts can freeze and eventually damage the gun. However, it is unlikely for this to occur since once the gas is released from the containing cylinder it instantly turns back into its gaseous state, and expands rapidly. It is doubtful whether the retained pressure behind the pellet before it begins to accelerate down the barrel is enough to keep the gas in a liquid form. Also, any gun that is expected to be exposed to the intense cold of de-pressurizing gas should have materials that can handle it.
Gas power tends to be used in airsoft pistols where size constraints make electric-powered mechanisms impractical. Other instances where gas is favored are where adjustable velocities are required or where a blowback feature is desired. A blowback feature is a mechanism which cycles a slide or bolt to better simulate a real firearm's operation. Because of the mechanical complexities involved with distributing and regulating gas, these guns have largely given way to electric guns for less specialized applications, however, they still remain favorable amongst some airsofters. They are not just limited to pistols; submachine gun airsoft replicas and sniper rifle airsoft replicas commonly use gas mechanisms. Whilst the submachine gun replicas typically feature a blowback mechanism similar to the pistol replicas, sniper rifle replicas usually omit the blowback mechanism in favor of reduced recoil and increased muzzle velocity.
Along with using gas to power guns, it is also applied for use in replica grenades. These grenades are both projectiles, fired from a grenade launcher such as the M203 or GP-25, or throwable. The shells work on the system of an internal piston, filled with gas. Either a series of pellets or in some cases a rubber or soft foam head is seated in or on top of the shell. When the pressure is released the projectile(s) are shot from the launcher sent downrange.
In the case of the throwable grenades, inside the grenade there is a similar piston to the one used in the shells, but is on a literal "timer" that allows the user to clear the area of effect. Pellets or powder act as the projectile in the case of these grenades. Currently both types of grenades are not very common, mostly because grenade launchers are quite expensive and the throwable grenades are not very reliable.
Classic airsoft guns are usually older variety airsoft guns which are gas powered. Unlike the gas pistols of today, they can run on either an internal tank using conventional airsoft gas or use an external CO2 tank much like a paintball gun. They generally cost more than the standard AEG but provide a more realistic approach to airsoft. Some models, such as those made by the Sun Project, feature a type of "recoil" provided by these guns. While these guns can become more powerful than AEGs, almost all users operate them at AEG power. These replicas often are fitted with a LRB (Long Range Barrel) instead of a traditional hop-up unit. The LRB turns the entire barrel into a hop-up system, giving a consistent spin on the BB. The rate of fire on these can be regulated by the amount of air being fed through the system, versus the motor strength of an AEG.