The 37 mm Automatic Gun, M4 (known as the T9 during development) was a 37mm recoil-operated autocannon designed by the Browning Arms Company and manufactured by Colt.
It entered military service in 1942, used in the Bell P-39 Airacobra and P-63 Kingcobra aircraft.
Design Details[]
An anti-aircraft weapon, the M4 had a muzzle velocity of 2,000 feet per second and a cyclic rate of 150 rounds per minute. It is normally loaded with M54 high-explosive tracer shells, but the weapon is also usable with M80 armor-piercing shells that could penetrate through an inch of armor plate at a distance of 500 yards. It was magazine-fed and could be fired either manually or by remote control using a solenoid mounted on the rear of the weapon.
Recoil and counter-recoil were controlled hydraulically via a piston and spring combination connected to the recoil mechanism and operating in an oil-filled recuperator cylinder that is mounted to the stationary trunnion block assembly. The recoil mechanism of the gun is comprised of the tube and tube extension, recuperator piston and piston rod, lock frame assembly, 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.
As the gun was originally designed, ammunition could be fed by a 5-round clip, a 15-round link belt, or an M6 non-disintegrating 30-round magazine, which was used exclusively in production. The M4 is fed ammunition only from the left-hand side of the weapon.
The M6 boasted an oval-shaped framework (nicknamed a "horsecollar magazine" for its shape) providing a track for the endless belt.
Initial loading and cocking of the weapon was done manually. A safety feature incorporated in the design of the trigger mechanism prevented firing a round until the breechblock assembly was put into 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 help 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The lock frame was retracted by recoil action during automatic firing, and is forced forward by the driving springs.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 breech block, fire the round by means of the hammer striking the firing pin, extract the cartridge case from the chamber, and operate the ejector.
By absorbing the energy of the lock frame, the back plate assembly 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 an incoming round. The springs then drove the lock frame assembly forward to operate the ejector, chamber the round, and raise the breech block.
Initial extraction occurred during recoil. Extraction, ejection, feeding and loading were done during counter-recoil. If the trigger was held in the firing position, the gun would continue to fire automatically until the magazine was empty.
Military Service[]
U.S. Air Forces[]
The M4 proved to be particularly unpopular with U.S. Air Force pilots, due to the weapon's drooping trajectory.
The only standard aircraft in service at the time to use the M4 was the Bell P-39 Airacobra and the P-63 Kingcobra.
The experimental XP-58 Chain Lightning, a larger, more heavily armed version of the P-38 Lightning, was originally equipped with four M4s in its nose. Because pilots weren't used to the weapon's drooping arc of fire, the quadruple M4s were replaced by a single 75mm M5 cannon with two .50 caliber heavy machine guns.
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M4s were mounted on numerous U.S. Navy PT boats as deck guns, starting with the Solomon Islands campaign. Primary targets were landing barges used by enemy forces to move supplies down the island chain at night. Initially, they were taken out of crashed P-39 aircraft at Henderson Field. When they proved successful as anti-barge weapons, they were used in this role for the rest of the war.
M4s were initially mounted on a simple pedestal mount (often built at the front lines) with the standard M6 magazine being used. Later, an improved pedestal mount was designed for original equipment mountings on the boats. Handgrips of several configurations were used, with various sighting systems being tried out. Most PT boat gunners used tracer rounds to sight the fall of their shots. Beginning in 1944, the M9 cannon was installed at the builders' boatyard as standard equipment.
Soviet Air Forces[]
In World War II, the U.S. supplied the Soviet Air Forces with M4-equipped P-39 and P-63 planes on lend-lease with 1,232,991 rounds of M54 ammunition. Primarily used for air-to-air combat on the Eastern Front, the M4 was sometimes used against soft ground targets.
The Soviets did not use the P-39 for tank-busting, as the U.S. did not supply M80 rounds with the planes. Soviet pilots, though off-put by the M4's slow cyclic rate and the low magazine capacity, had appreciated its reliability.
U.S.-built contemporaries and successors[]
- 37mm M10 autocannon - Minor upgrade of the M4 fed by a metallic disintegrating link belt with a slightly higher cyclic rate of around 165 rounds per minute. The belt made it possible to store more ammunition for the weapon on a host aircraft, for a new total of 58 rounds. It replaced the M4 in aircraft, starting with the A-9 model of the Bell P-63 Kingcobra.
- 37mm M9 autocannon - A derivative of the M1A2 flak gun using the longer and more powerful 37×223mmR cartridge. The M9 had a 50% higher muzzle velocity than the M4 (at 3,000 ft/s) from a 78-inch barrel, and was twice as heavy at 120 lbs for the barrel alone. The complete M9 weighed a very heavy 405 lbs compared to the 213 lbs of the M4. It has the same cyclic rate. Very little is known about the weapon's deployment, other than its only confirmed use in U.S. Navy PT boats.