Abstract: A breech mechanism having a breech block and an obturator for an automatic weapon for firing with a high firing frequency and high gas pressure without inhibiting the safety of the weapon against jamming or misfiring.The obturator is pivotally mounted about a first axis which is normal with respect to the longitudinal axis of the gun barrel to pivot about an axis of 90.degree. from a closed firing position to an open loading position and vice-versa. A pair of control cams are operatively connected via a pair of projecting pins to said obturator in said breech aperture. The breech block has a pair of laterally projecting pins which are adapted to slidingly guide the control cams during their pivotal movement. A pair of biasing members are pivotally connected at one of their ends to opposite walls of the breech aperture and at their other ends to the pair of control cams so as to bias the obturator either into the closed firing position or the open loading position.