"These guns are capable of automatic and semi-automatic operation. The most common gases used are ""green gas"" and Propane which requires an adapter. Less commonly used gases include ""red gas"" (which is actually HCFC-22), CO2 and nitrogen/high pressure air.
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. 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 However, these guns have largely been superseded by the newer and more versatile AEGs, or automatic electric guns.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."
