air exhaust plenum 28 extends substantially completely across the width of the enclosure from one side wall 18 to the other side wall 20 and is connected to an air duct 32 a plurality of preset constant volume air valves 34 are in the exhaust air plenum 28 at its opening into the enclosure 12 to maintain a constant volume of air flow into the exhaust air plenum as the air being exhausted from the enclosure may vary from time to time in practice it has been found satisfactory to size the exhaust air plenum to move various volumes such as for example 15 000 cubic feet of air per minute with a velocity of about 630 feet per minute per linear foot of exhaust plenum across the width of the enclosure this insures that a negative air pressure will be maintained with the firing range fig1 and 3 illustrates one advantageous embodiment of the air supply plenum 30 which comprises an air impervious wall structure 36 spaced from and generally parallel to the uprange end wall 14 and an air pervious diffusing wall structure 38 spaced downrange from and generally parallel to the air impervious wall structure 36 the air supply plenum 30 being defined between the air impervious wall structure 36 and the air pervious diffusing wall structure 38 the area uprange of the air pervious wall structure.