Gas Airsoft Guns use pressurized Gas to propel Airsoft Pellets. These Airsoft Guns are capable of Automatic and Semi-Automatic operation. The most common Gas used is propane (usually referred to as "green gas" by Airsoft Players). Less commonly used gases include CO2 and nitrogen. Gas power tends to be used in Airsoft Pistols where size constraints make Electric Powered Airsoft Gun mechanisms impractical. Other instances where Gas is favored are where adjustable velocities are required or where a Blowback feature is desired. A blow back 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 Gas Airsoft Guns have largely given way to Electric Airsoft Guns for less specialized applications.