1. IN APPARATUS FOR SIMULATING THE FIRING OF A WEAPON, THE COMBINATION OF: (A) MEANS DEFINING A GAS PRESSURE CHAMBER HAVING (1) AN INLET CONNECTABLE WITH A SOURCE OF GAS UNDER PRESSURE AND (2) A BORE THROUGH ONE WALL OF THE CHAMBER, SPACED FROM SAID INLET; (B) A DISC MOUNTED FOR ROTATION ABOUT AN AXIS SPACED FROM SAID BORE AND SUBSTANTIALLY PARALLEL THERETO, SAID DISC OVERLYING THE OUTER END OF THE BORE AND HAVING AN OPENING THEREIN WHICH SUBSTANTIALLY ALIGNS WITH THE BORE IN ONE ROTATIONAL POSITION OF THE DISC; (C) AN AXIALLY MOVABLE PRESSURE RESPONSIVE SEALING ELEMENT IN THE BORE HAVING A HOLE EXTENDING AXIALLY THERETHROUGH, SAID SEALING ELEMENT HAVING AN ANNULAR SEALING SURFACE ON ITS EXTERIOR, SURROUNDING THE HOLE THEREIN, WHICH CAN ENGAGE THE ADJACENT FACE OF THE DISC TO PROVIDE A SEAL THAT DOES NOT INTERFERE WITH ROTATION OF THE DISC BUT PREVENTS ESCAPE OF PRESSURIZED GAS FROM THE CHAMBER WHEN THE DISC IS OUT OF ITS SAID ROTATIONAL POSITION, SAID SEALING ELEMENT BEING ADAPTED TO BE BIASED INTO SUCH ENGAGEMENT WITH THE DISC BY PRESSURIZED GAS IN THE CHAMBER; AND (D) MEANS FOR ROTATING THE DISC SO AS TO CYCLICALLY BRING SAID OPENING THEREIN INTO ALIGNMENT WITH THE BORE AND HENCE INTO REGISTER WITH THE HOLE IN THE SEALING EEMENT SO THAT PRESSURIZED GAS CAN THUS BE ABRUPTLY RELEASED FROM THE CHAMBER TO PRODUCE A SOUND SIMULATING THE FIRING OF A WEAPON.