The 37 mm Automatic Gun, M4, known as the T9 during development, was a 37 mm (1.46 in) auto cannon designed by John Browning and used in the Bell P-39 Airacobra and P-63 Kingcobra fighters, as well as experimentally on other designs. It provided interceptors with a weapon that could shoot down any bomber with as little as one hit. It was a compact design with a relatively low muzzle velocity and rate of fire.
Designed primarily as an anti-aircraft weapon, the gun had a muzzle velocity of 2,000 ft/s (610 m/s) and a cyclic rate of 150 rounds per minute. It was normally loaded with high-explosive shells, but could also be loaded with the M80 armor-piercing shell, which could penetrate 1 in (25 mm) of armor plate at 500 yd (460 m). It was magazine-fed and could be fired manually or by remote control through a solenoid mounted on the rear of the gun. Recoil and counter-recoil were controlled hydraulically by means of a piston and spring combination connected to the recoiling parts and operating in an oil-filled recuperator cylinder mounted to the stationary trunnion block assembly. The recoiling parts of the gun included the tube and tube extension, the recuperator piston and piston rod, the lock frame assembly, the driving spring assemblies, and the breechblock assembly. The non recoiling parts included the trunnion block group, the feed box and feeding mechanism, the recuperator cylinder and bushing, the back plate group, and the manual charger assembly.
Ammunition was issued in the form of fixed rounds, consisting of H.E. shell, M54, with P.D. fuse, M56; practice shell, M55A1, with dummy fuse, M50; and A.P. shot, M80.
The rapid strides in aircraft protection made it necessary to develop an aircraft weapon that would fire projectiles with greater explosive and armor-piercing qualities than smaller caliber weapons. As a result, the 37 mm (1.46 in) automatic gun, M4, was developed and standardized for aircraft use.
The 37 mm gun (1.46 in), M4, used the same high-explosive (M54) and practice (M55A1) projectiles as the 37 mm (1.46 in) antiaircraft gun, M1A2, but different cartridge cases are necessary due to the larger chamber of the M4 gun.
However, the overall length of the armor-piercing projectiles, M51 and M74, which were used in the M3A1, M5A1 and M6 tank and antitank guns, was too great to permit their use in the M4 gun and the 37 mm (1.46 in) armor-piercing shot, M80, was developed and standardized.
High-explosive shell, 37 mm, M54 standard
This shell used the M56 point detonating fuse. The complete round weighs 1.99 lb (900 g); as fired, the projectile weighs 1.34 lb (608 g). The 0.16 lb (70 g) charge of M2 powder is a Hercules NG formula of single perforated grains with 0.030 in (0.76 mm) web and gives the projectile the prescribed muzzle velocity of 2,000 ft/s (610 m/s).
The M54 used a shell-destroying tracer in addition to the point-detonating fuze. The tracer had a burning time of three seconds at the end of which it set off an igniting relay charge of 1.68 gr (0.109 g) of Grade A-5 Army Black Powder which ignited a relay pellet that detonated the charge, destroying the shell before ground impact.
The bursting charge of tetryl weighed 0.10 lb (45 g), and the alternate Composition "A" charge weighs 0.105 lb (48 g). The tetryl loading consisted of a 200 gr (13 g) tetryl pellet pressed into the shell cavity under 9,000-10,000 psi (60-70 MPa) pressure and the remainder of the charge of two equal increments pressed under approximately 9,000 psi (60 MPa) pressure. The Composition "A" bursting charge is loaded in the same manner as the tetryl charge, except that the relay pellet with the Composition "A" weighs 36 gr (2.3 g) as against 23 gr (1.5 g) for the pellet used with the tetryl load.
Practice shell, 37 mm, M55A1 standard
This shell was the high-explosive shell modified slightly for practice purposes. It contained a red tracer and a dummy fuse (M50, M50B1, M50B2 or M50B3). The M50 dummy fuze was made from a plastic composition and the M50B1, M50B2 and M50B3 fuses were made from low carbon steel machined to give the same contour and weight as the point-detonating fuse, M56, used with the M54 projectile.
As used in the M4, the complete round weighed 1.99 lb (900 g), and as fired the shell weighed 1.34 lb (610 g). The 0.16 lb (70 g) charge of M2 powder was Hercules NG formula of single perforated grains with a 0.030 in (0.76 mm) web and gave the prescribed muzzle velocity of 2,000 ft/s (610 m/s).
Armor-piercing shot, 37 mm, M80 standard
The AP shot was a monoblock projectile with a tracer element of three seconds burning time. It did not need a fuse or bursting charge. The weight of the complete round was 2.31 lb (1.05 kg), the weight of the AP shot was 1.66 lb (750 g). The propelling charge was 0.15 lb (78 g) of M2 powder of a Hercules NG formula with a single-perforated grain and 0.030 in (0.76 mm) web.