Warfare Equipment

The 37 mm Automatic Gun, M4, known as the T9 during development, was a 37 mm (1.46 in) autocannon 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.

Design

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 nonrecoiling 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.

Feeding mechanism

As the gun was originally designed, ammunition could be fed by a 5-round clip, a 15-round link belt, or a non-disintegrating 30-round endless belt magazine. The 30-round endless belt version was used exclusively in production. The M4 gun fed only from the left.

The 30-round Endless Belt Magazine, M6, was an oval-shaped framework (nicknamed a "horsecollar magazine", from its shape) providing a track for the endless belt. The articulated link belt actually contained 33 rounds: consisting of 30 HE and/or AP shells and three tracer rounds (one at the end of each 10-shell section) to improve accuracy.

Firing cycle

Initial loading and cocking of the gun were accomplished manually. A safety feature incorporated in the design of the trigger mechanism prevented firing the round until the breech-block assembly was in the battery position.

The breech was locked and unlocked by recoil action which brings the operating level guide pins against cams to raise and lower the breechblock. The function of the breechblock was to assist in the final chambering of the round, close the breech, and actuate the trigger trip. It also provided a mounting for the firing pin.

The lock frame, during automatic firing, was retracted by recoil action and is forced forward by the driving springs. The major function of the lock frame assembly was to force the cartridge into the chamber, actuate the breechblock, fire the round by means of the hammer striking the firing pin, extract the cartridge case from the chamber, and operate the ejector.

The back plate assembly, by absorbing the energy of the lock frame, reduced the shock against the carrier pin as the lock frame was hatched to the rear.

The driving spring assemblies held the lock frame against the carrier dog until the carrier was released by carrier catch which was pivoted by the incoming round. The springs then drove the lock frame assembly forward to operate the ejector, chamber the round and raise the breechblock.

Initial extraction occurred during recoil. Extraction, ejection, feeding and loading were accomplished during counter-recoil. If the trigger was held in the firing position, the gun would continue to fire automatically until the magazine was empty.

Ammunition

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.

Use

The only aircraft to see service use of the M4 were the Bell P-39 Airacobra and the derivative P-63 Kingcobra, both of which saw action with the Soviet Air Forces on the Eastern Front. The U.S did not supply M80 armor-piercing rounds for the autocannons of Soviet P-39s—instead, the Soviets received 1,232,991 M54 high-explosive rounds which the Soviets sometimes used against soft ground targets but primarily for air-to-air combat. The Soviets did not use the P-39 for tank-busting.

The M4 37 mm (1.46 in) automatic cannon was mounted on numerous U.S. Navy PT boats as deck guns, beginning with the Solomon Islands campaign. At first, they were cannibalized from crashed P-39s at Henderson field, and due to their success as an anti-barge weapon were used for the rest of the war. Beginning in 1944, the M9 model 37 mm (1.46 in) cannon was installed at the builders' boatyard as standard equipment. The M4s were initially mounted on a simple pedestal mount (often built at the front lines) with the standard horseshoe endless-belt feed being used. Later, an improved pedestal mount was designed for original equipment mountings on the boats. Handgrips of several configurations were used with various sights being tried. Most PT boat gunners used tracers to sight the fall of their shot. Primary targets were the landing barges being used to move supplies down the island chain at night.

SCREENSHOOT