A breech closure for a barrel-type firearm is described, in which a flow passage for an igniting jet from an igniting cartridge held in a magazine extends through a breechblock. The breechblock is formed on that side which faces the magazine with a cylindrical recess, which is coaxial to the flow passage for the igniting jet and contains a metal sealing cup, the side wall of which contacts the peripheral surface of the recess and which has a bottom, which is formed with a central through opening for receiving the igniting jet and constitutes a conical disk spring, which bears on the contacting surface of the magazine around said through opening, the rim of which is enlarged to form a bead on the inside of the cup.