Gas powered airsoft guns are one type of BB gun that functions on—as the name suggests—gas. Other varieties include spring powered and electric powered airsoft guns. To use the gas powered gun, one of an assortment of gases are either injected into a reservoir within the gun or attached to an exterior tank called the magazine or clip. Low temperature carbon dioxide, environment safe Freon, and propane are the most popular types of gas to use with the gas powered airsoft gun. The gas powered airsoft gun comes in two models: blowback and non-blowback models. In the blowback model, gas pushes both the slide, which is on top of the gun, and the BB forward. BB's are typically 6mm spherical pellets made of solid plastic. In the non-blowback model, the slide stays fixed and the gas pushes the BB forward. This model is more efficient because it requires less gas to push the BB. Created in Japan in the 1970s because real guns are prohibited, the gas powered airsoft gun's use is for recreational sport. Considered a military-style mock weapon, players use these types of guns to participate in military-style mock combat with mock tactics. While used for recreation, gas guns have an advanced design and can still cause danger if used improperly. Only responsible adults should use gas powered airsoft guns—or any other airsoft gun—and they should wear both eye and mouth protection when participating in a game of friendly fire.