Gas operation (firearms)
Gas-operation is a system of operation used to provide energy to operate autoloading firearms. In gas operation, a portion of high-pressure gas from the cartridge being fired is used to power a mechanism to extract the spent case and insert a new cartridge into the chamber. Energy from the gas is harnessed through either a port in the barrel or a trap at the muzzle. This high-pressure gas impinges on a surface such as a piston head to provide motion for unlocking of the action, extraction of the spent case, ejection, cocking of the hammer or striker, chambering of a fresh cartridge, and locking of the action.
Direct impingement is a form of gas operation that does away with the piston completely. The bolt is designed in such a way so that the gas acts directly upon the bolt (hence the name) to cycle the action.