Updated: Aug 31
In July of 2021 we managed to acquire an M4 Auto Cannon, the main gun of the WW2 area P-39 Airacobra fighter plane. The gun was a used weapon that had been cut to be used as a training demonstrator and probably shipped to Russia during lend lease as is evident by the usual Russia black paint. The over all condition of the weapon was very good given its age and obvious rough storage conditions. What was initially thought to be rust was in fact dried oil and hence the condition of the metal was actually very well preserved in most places with the notable exception of the recuperator tube sleeve which had rusted almost completely away.
Background information on the weapon system
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 inch (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 mechanism and operating in an oil-filled re-cooperator cylinder mounted to the stationary trunnion block assembly. The recoiling mechanism of the gun included the tube and tube extension, recuperator piston and piston rod, lock frame assembly, driving spring assemblies, and the breech block 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 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 was given the designation M6; it had an oval-shaped framework (nicknamed a "horsecollar magazine" from its shape) providing a track for the endless belt.
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 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.
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 breech block.
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.
US Army Air Forces
P-39Q Airacobra weapons bay showing M4 cannons "horse-collar" drum magazine
The 37 mm cannon was disliked by pilots for its low muzzle velocity, resulting in more pronounced drop than other contemporary weapons.
The only standard aircraft armed with the M4 to see service were the Bell P-39 Airacobra and the derivative P-63 Kingcobra. It was used as a limited standard aircraft in North Africa and the Pacific theater by the USAAF and Allied air forces.
The experimental XP-58 "Chain Lightning" was a larger, more heavily armed version of the P-38 Lightning equipped with quadruple M4 cannons instead of M2 heavy machine guns in its nose. Its original purpose (like the German Bf 110) was to destroy formations of bombers, but it was later re-envisioned as a ground-attack plane. The more pronounced ballistic trajectory was unfamiliar for American pilots, and the four M4s were replaced with a single 75mm M5 cannon and a pair of .50 heavy machine guns.
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. Primary targets were the landing barges being used to move supplies down the island chain at night. 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. 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. Hand grips of several configurations were used with various sights being tried. Most PT boat gunners used tracers to sight the fall of their shot. Beginning in 1944, the M9 model 37 mm (1.46 in) cannon was installed at the builders' boatyard as standard equipment.
Soviet Air Forces
During World War II the United States supplied the Soviet Air Forces with the M4-equipped P-39 Airacobra and P-63 King Cobra. The U.S did not supply M80 armor-piercing rounds for these Lend-Lease aircraft—instead, the Soviets received 1,232,991 M54 high-explosive rounds. The M4 was sometimes employed against soft ground targets on the Eastern Front but was primarily used in air-to-air combat, in which role it was highly effective. The Soviets did not use the P-39 for tank-busting. Soviet pilots appreciated the M4's reliability but complained of its low rate of fire (three rounds per second) and small magazine size (30 rounds).
The first stage in any restoration is to break down all the components to their smallest size, not only for ease of cleaning but in this case to pare down some of this weapon’s 239 pound bulk and make it easier to move around. It was very lucky that both USAAC training films for the weapon are still available on the internet and an original training manual was acquired as well. Due to the deteriorated condition of the recuperator tube the original goal of having a parts free moving would have to be abandoned. Furthermore, the Barrel Extension could not be removed due to the cuts made in it during the cutaway trainer manufacturing process.
One by one all parts (where possible) were removed de-greased in our chemical parts washer and then either polished by hand or sandblasted. Many proof marks became evident during this process. The most prevalent being OSMC (Oldsmobile Motor Company) the primary contractor on this particular gun.
The finish of the gun was originally green tint Parkerizing but has been painted black numerous times. It was decided to leave the internal workings in their natural finish, paint the receiver and barrel black again and then restore the red cutaway lines by hand.