1. AN EXPLOSIVE-OPERATED FIREARM COMPRISING A RIFLED BARREL OPEN AT ITS DISCHARGE END AND SEALED AT ITS LOADING END BY MEANS DEFINING A FIRING CHAMBER, MEANS DEFINING A BREECH OPENING ADAPTED TO ADMIT THROUGH SAID BREECH OPENING AND INTO SAID BARREL AT ITS LOADING END A PROJECTILE TO BE FIRED FROM SAID FIREARM, A VALVE SEAT DISPOSED ON AN INTERNAL WALL OF SAID FIRING CHAMBER AROUND SAID BREECH OPENING BETWEEN SAID BREECH-BLOCK ADAPTED TO MATE FIRING CHAMBER, A MOVABLE BREECH-BLOCK ADAPTED TO MATE WITH AND SEAL AGAINST SAID VALVE SEAT FROM INSIDE THE FIRING CHAMBER, AN OPERATING MEMBER OF CROSS-SECTIONAL AREA SMALLER THAN SAID BREECH OPENING CARRYING SAID BREECH BLOCK AND EXTENDING THEREFROM THROUGH SAID BREECH OPENING TO A MOUNT DISPOSED EXTERNALLY OF SAID FIRING CHAMBER, DRIVING MEANS NORMALLY DISPOSED EXTERNALLY OF SAID FIRING CHAMBER FOR RECIPROCATING SAID OPERATING MEMBER ON SAID MOUNT TO CARRY SAID BREECH BLOCK AWAY FROM SIAD VALVE SEAT AND INTO SAID FIRING CHAMBER IN ORDER TO OPEN SAID BREECH OPENING TO PERMIT LOADING OF A PROJECTILE THERETHROUGH INTO THE LOADING END OF SAID BARREL, MEANS NORMALLY URGING SAID BREECH BLOCK AGAINST SAID SEAT, AND MEANS FOR FIRING AND EXPLOSIVE CHARGE WITHIN SAID FIRING CHAMBER, WHEREBY ON EXPLOSION OF AN EXPLOSIVE CHARGE WITHIN SAID FIRING CHAMBER, SAID BREECH BLOCK IS FURTHER URGED AGAINST SAID SEAT BY THE PRESSURE OF THE EXPLOSION, THEREBY UTILIZING THE FIRING CHAMBER PRESSURE TO COOPERATE IN MAINTAINING A GAS-TIGHT SEAL OF THE BREECH OPENING.