M4 cannon

M4 cannon

Infobox Weapon
name= 37 mm Automatic Gun, M4

origin= USA
weight= 213 lb (97 kg)
length= 89.5 in (2.27 m)
caliber=37 mm (1.46 in)
rate= 150 rpm
velocity= convert|2000|ft/s|abbr=on
feed= 30 round magazine

The 37 mm Automatic Gun, M4, known as the T9 during development, was a 37 mm autocannon used in the Bell P-39 and P-63 fighters, as well as experimentally on other designs. It was designed to provide interceptors with a weapon that was guaranteed to destroy any bomber with as little as one hit. It was a compact design, but it had a slow muzzle velocity and a very low rate of fire.

Designed primarily as an anti-aircraft weapon, the gun had a muzzle velocity of convert|2000|ft/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 one inch (25 mm) of armor plate at convert|500|yd|m|-1. It is magazine-fed and may be fired manually or by remote control through a solenoid mounted on the rear of the gun.

Recoil and counter-recoil are 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 include 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 include the trunnion block group, the feed box and feeding mechanism, the recuperator cylinder and bushing, the back plate group, and the manual charger assembly.


*Type: single-barrel semi-automatic cannon
*Caliber: 37 mm (1.46 in)
*Projectile weight: 608 g (1.34 lb)
*Weight of gun: 213 lb (97 kg)
*Weight of 30-round magazine, empty: 5.5 lb (2.5 kg)
*Length of gun, overall: 89.5 in (2.27 m)
*Operation: gas
*Muzzle velocity: 610 m/s (2,000 ft/s)
*Maximum powder pressure: 23,200 psi (160 MPa)
*Length of recoil: 9⅝ in (245 mm)
*Rate of fire: 150 rds/min

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 (horsecollar) magazine. The 30-round endless belt was used exclusively in production. The 30-round endless belt Magazine, M6, is an oval-shaped framework providing a track for the endless belt. The articulated link belt contains 33 rounds, although only 30 rounds are ordinarily loaded into the magazine. The M4 gun feeds only from the left.

Firing cycle

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

The breech is 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 is to assist in the final chambering of the round, close the breech, and actuate the trigger trip. It also provides a mounting for the firing pin.

The lock frame, during automatic firing, is retracted by recoil action and is forced forward by the driving springs. The major function of the lock frame assembly is 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, reduces the shock against the carrier pin as the lock frame is hatched to the rear.

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

Initial extraction occurs during recoil. Extraction, ejection, feeding and loading are accomplished during counter-recoil. If the trigger is held in the firing position, the gun will continue to fire automatically until the magazine is empty.


Ammunition is 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 automatic gun, M4, was developed and standardized for aircraft use.

The 37 mm gun, M4, uses the same high-explosive (M54) and practice (M55A1) projectiles as the 37 mm 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 are 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 armor-piercing shot, M80, was developed and standardized.

;High-explosive shell, 37 mm, M54 standardThis shell uses the point detonating fuse, M56. The complete round weighs 1.99 pounds (900 g); as fired, the projectile weighs 1.34 pounds (608 g). The 0.16-pound (70 g) charge of M2 powder is a Hercules NG formula of single perforated grains with 0.030 inch (0.76 mm) web and gives the projectile the prescribed muzzle velocity of convert|2000|ft/s|m/s|-2.

The M54 features the shell-destroying tracer in addition to the point-detonating fuze. The tracer, which has a burning time of three seconds, sets off an igniting relay charge of convert|1.68|gr|g of Grade A-5 Army Black Powder which ignites a relay pellet to detonate the charge and destroy the shell before ground impact.

The bursting charge of tetryl weighs 0.10 pound (45 g), and the alternate Composition “A” charge weighs 0.105 pound (48 g). The tetryl loading consists of a 200 grain (13 g) tetryl pellet pressed into the shell cavity under 9,000 to 10,000 psi (60 to 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 convert|36|gr|g as against convert|23|gr|g for the pellet used with the tetryl load.

;Practice shell, 37 mm, M55A1 standard

This shell is the high-explosive shell modified slightly for practice purposes. It contains a red tracer and a dummy fuse, M50, M50B1, M50B2 or M50B3. The M50 dummy fuze is made from a plastic composition and M50B1, M50B2 and M50B3 fuses are 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 automatic gun, M4, the complete round weighs 1.99 pounds, and as fired the shell weighs 1.34 pounds (610 g). The 0.16 pound (70 g) charge of M2 powder is Hercules NG formula of single perforated grains with a 0.030 inch (0.76 mm) web and gives the prescribed muzzle velocity of convert|2000|ft/s|m/s|-2.

;Armor-piercing shot, 37 mm, M80 standard

The weight of the complete round is 2.31 pounds (1.05 kg), the weight of the A.P. shot is 1.66 pounds (750 g). The propelling charge is 0.15 pound (78 g) of M2 powder of a Hercules NG formula with a single-perforated grain and 0.030 inch (0.76 mm) web. The shot is a monoblock projectile with a tracer element of three seconds burning time and has no fuse or bursting charge.


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 Soviets on the Eastern Front.

Numerous Photographs of M4 37mm Cannon mounted on PT boats in the Salomon Islands and New Guinea areas during WW II. Use confirmed by me in Person with Support commander who serviced PT-109 and her sister Boats. PT-109 used a 37mm Anti Tank gun Minus it's wheels strapped to the Foredeck. The M4 Cannons were usually Mounted on a shop built simple Pedestal mount with the stamdard horseshoe endless belt feed being used. Handgrips of several configurations have been seen with various sights being tried. Primary targets were the landing barges being used to move supplies down the island chain at night.

ee also

;Comparable weapons
*Nudelman N-37
*Vickers S gun

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