Gas airsoft pistols utilize a green gas or CO2 chamber to propel the airsoft ammo through the air. The most popular type of gas airsoft pistols are the gas blowback pistols (GBB), which have a life like slider retraction when fired. This makes the gas airsoft pistol recoil very much like a real gun. The non blowback pistols do not have such a mechanism, and thus are generally more economical than the gas blowback pistols.