Aircraft technical Basics: TM 1-409, Aircraft Armament and Pyrotechnics, 1941: VII. - Aircraft Machine Guns, Caliber .50

Section VII - AIRCRAFT MACHINE GUNS, CALIBER .50

41. General.-The Browning machine gun, caliber .50, M2, aircraft, is a recoil-operated, belt-fed, air-cooled machine gun. The metallic link, disintegrating belt, is used in all firing of the gun. This gun is designed for both fixed and flexible use, and by properly repositioning some of the component parts the gun may be fed from either the right- or left-hand side as desired.

a. Types.-(1) Fixed gun.-The fixed gun is designed for installation on or adjacent to the airplane engine; or for mounting in or on the wings. It is provided with an operating slide which connects with the bolt by means of the bolt stud. The operating slide is provided for retraction of the breech mechanism by hand and for use in loading, unloading, and reduction of stoppages in firing. It can be drawn back to its full extent or to a position with the bolt not fully home and the bolt retained there by engaging the slot provided in the lower surface of the operating slide on the operating slide rear guide.

(a) The fixed gun is normally assembled with a backplate having a horizontal buffer, although it may have a vertical buffer backplate due to limitation in mounting space.

(b) The fixed type of gun when mounted on or adjacent to the engine is fired by means of a synchronized mechanical trigger motor attached to the gun receiver. This mechanism causes the gun to deliver semiautomatic fire. When wing mounted, the gun is fired through a remote controlled solenoid attached to the gun receiver, the gun firing delivering automatic fire. The synchronizing mechanism, including the trigger motor, and the remote control mechanism, including the solenoid, are supplied by the Air Corps.

(2) Flexible gun.-The flexible gun is installed in the fuselage of the airplane and is provided with a retracting slide located on the right side of the gun. The retracting slide is provided for retraction of the breech mechanism by hand and for use in loading, unloading, and reduction of stoppages in firing. It is connected with the bolt by means of the retracting slide bolt stud. The retracting slide handle remains stationary and in a forward position while the gun is firing, thus eliminating all moving parts outside of the receiver.

(a) The flexible gun is provided with a backplate having a horizontal buffer, double spade grips, and a hand trigger.

(b) The gun is fired by pressing down the trigger. The operation of the gun is fully automatic ; that is, it is self-operative within the limit of capacity of the ammunition belt by simply keeping the trigger depressed.

b. Cooling.-Aircraft machine guns are air-cooled, and as mounted at present, there is very little difference in the cooling whether the gun is fired on the ground or in the air.

(1) With the standard 36-inch caliber .50, M2, barrel weighing 9½ pounds, a maximum burst of 75 rounds may be fired from either a flexible or a wing gun. Approximately 1 minute after firing a 75-round burst, firing may be resumed and 20 rounds fired and repeated each minute thereafter.

(2) For synchronized guns where the heat from the engine pre-heats the gun, the burst should be limited to 50 rounds, and after approximately 1 minute, firing may be resumed and 15 to 20 rounds fired and repeated each minute thereafter.

(3) The long burst will heat the barrel to the maximum permissible temperature, and repeated firing after 1 minute delay with a reduced number of rounds per minute will maintain the barrel at. the high temperature. Thus the initial burst of 50 to 75 rounds or a 50- to 75-round burst followed by firing 20 rounds for each succeeding minute requires a cooling time or cessation of fire for approximately 15 minutes before the long burst can be repeated.

(4) If long bursts are not fired, approximately 25 rounds may be fired each minute over long periods.

c. Mounting.-(1) Fixed gun-The mounting is for fixed synchronized fire, and the mount is arranged to locate the gun as far forward of the pilot as possible, usually within the cowling. The mount is rigid with respect to the airplane, and the gun fires in a line approximately parallel to the direction of flight and directly through the path of propeller rotation. The gunfire is controlled so as to miss the propeller blades by action of the synchronizing mechanism, which is the device operating between the engine and the gun for this purpose. The wing mounting is dependent upon the design of the wings and may be interior or exterior.

(2) Flexible gun-The gun is mounted on a flexible mount which permits a larger angular field of fire in both azimuth and elevation. The gun is fired by hand.

d. Sights.-(1) The sights for the gun are carried as airplane equipment rather than as part of the gun itself and are supplied by the Air Corps.

(2) Sights for the fixed gun, when used, are normally attached to the airplane. Sights for the flexible gun may be attached to the two sight bases which are mounted on the barrel jacket of the gun. They can readily be installed or detached.

(3) Complete information relative to sights is available in publications of the Air Corps.

42. Stoppages.-The instructions on stoppages are identical for both caliber .30 and caliber .50 machine guns. For these instructions, see paragraph 39.

43. Care and preservation.-a. General.-The importance of a thorough knowledge of how to care for and clean the machine gun cannot be over emphasized. The kind of attention given to a weapon of this type determines largely whether or not it will shoot accurately and function properly when needed. The bore and chamber must be kept in perfect condition for accurate shooting. Also it is just as important that the receiver and moving parts be kept clean, lubricated, and in perfect condition for efficient functioning.

b. Method of filling oil buffer-The oil buffer should be kept filled with machine-gun recoil oil (U. S. A. Spec. No. 2-77). In an emergency, any light recoil oil may be used, adding glycerin in freezing temperatures if necessary.

e. Points to be observed before a flight.-The following points must be observed before leaving the ground :

(1) Wipe bore and chamber of gun barrel.

(2) See that adjusting screws are screwed in tight against buffer disks in backplate.

(3) Test functioning of gun by hand, using dummy cartridges.

(4) Test functioning of operating slide (fixed gun only) or retracting slide (flexible gun only).

(5) Oil carefully.

(6) See that sight bases are clamped securely in place (flexible gun only).

(7) Make sure that ammunition belt is in good condition and properly loaded.

(8) Carefully place belt in ammunition box and see that metallic belt link chutes are in good condition with double end leading for either right or left feed, and in proper alinement.

(9) Load gun partially or completely as directed.

d. Points to be observed after a flight.-The following points must be observed as soon after a flight as practicable :

(1) Unload gun completely and remove belt from ammunition chest.

(2) Clean bore and all working parts. If this cannot be done at once, oil carefully to prevent, rust.

(3) Release firing pin spring.

(4) The armorer must get a detailed account from the gunner or pilot of the action of the gun in the air. If stoppages have occurred, their cause must be determined and corrected immediately.

(5) At the first opportunity, dismount gun; clean, oil. and inspect all parts; make needed repairs and replacements.

(6) On assembling, cheek operation with dummy cartridges and release firing pin spring after insuring that functioning and adjustments are correct.

e. Care and cleaning of gun-The instructions on the care and cleaning are identical for both caliber .30 and caliber .50 machine guns. For these instructions see paragraph 40.

44. Aircraft cannon.-a. Hispano Suiza automatic cannon, caliber 20-mm.-(1) Description.-The 20-mm cannon is an automatic firing gun used as a defensive and offensive weapon against enemy aircraft. This cannon is mounted to fire through the propeller shaft, and in other fixed installations the gun is mounted in the wing and fires outside the arc of propeller rotation.

(2) Cleaning and preservation.-The same precautions should be observed for maintaining this gun as are observed for the other aircraft machine guns.

b. Browning aircraft automatic cannon, caliber 37-mm.-(1) Description-The 37-mm aircraft cannon was designed primarily for use against enemy aircraft. It can be used in both fixed flexible and semiflexible installations. In fixed installations the cannon is mounted to fire through the propeller shaft. In flexible and semiflexible installations the place of mounting is such that. the line of fire will be outside the arc of propeller rotation. For more complete and detailed information on this cannon, see TM 9-240.

(2) Cleaning and preservation-The same precautions should be observed for maintaining this gun as are observed for the other aircraft machine guns.

NOTE.-The projectiles used in both the 37-mm and the 20-mm cannon are loaded with high explosive, and are designed to explode upon impact. The projectiles used in both the 37-mm and the 20-mm cannon are loaded with high explosive and a supersensitive fuze designed to explode on impact. Ther-fore, it is impracticable to use them in a synchronized installation. Extreme care must be exercised when handling this ammunition, and under no circumstances will the projectiles be removed from the cartridge case or tampered with for any reason. Cannon are used on fighter aircraft because of increased fire power, range, and increased effectiveness against armored aircraft.