Abstract: A firearm formed by a housing, in which an assembly of a carriage and a barrel with a muzzle brake, and a breech block are slidingly supported, wherein the breech block is equipped with a return spring that is supported on a rear support surface of the housing, has a counter weight slidingly disposed in a longitudinal space located between the housing and the carriage and has a support surface which leans against a rear end of the return spring, the said counter weight being seated in its front position on a front stop surface of the carriage.