most gas powered pistols. In addition, most AEPs are constructed almost entirely of plastic and have a light, toy-like feeling to them. Gas-powered Gas blowback Gas-powered airsoft guns use the pneumatic potential energy stored within compressed gas to drive the shooting mechanism, and thus operate according to an entirely different design principle to spring- or electrically powered airsoft guns. The most common type seen is the gas blowback (GBB) guns. These gas guns use an internal canister (usually within the magazine) that upon trigger-pull releases the prefilled bottled gas via a series of valves to propel the pellet and generate a blowback, which simulates recoil and cyclically loads the next shot. They are capable of both automatic and semi-automatic operation. The most common gases used are "green gas" and propane which requires an adaptor. HFC-134a is also commonly used, particularly with pistols which have plastic sliders due to the lower pressure giving a smaller chance of damage to the weaker slide. Less commonly used gases include "red gas" (which is actually HCFC-22), CO2 and nitrogen/high-pressure air. However it is unlawful to use HCFC-22 as a propellant in the US, as it is a Class II ozone-depleting substance and its use as an 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 feeding system with a majority of the configurations containing two tanks, one containing the R-12 and one used as an expansion tank, and the gun itself. R-12 was a commonly used refrigerant for car air conditioning and refrigerators, and 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