The advent of aircraft as effective tools of war in World War 1 prompted the need for ground-based artillery to track and ultimately engage and destroy such airborne enemy targets. The United States Army began work on such a weapon but the end of the war signaled a massive slowdown in the requirement and the program proceeded at a snail's pace until a new world war seemed all but inevitable with Hitler, Mussolini and Stalin moving on Europe and North Africa and the Japanese conquests of territories in the Pacific. The US Army dove back into the search for a modern anti-aircraft solution in 1938 and the result became the "4.7-inch M1' - a cannon of 120mm caliber specifically designed to help defeat the new-found threats posed by the Axis air powers - namely metal-skinned monoplane aircraft types that were designed with much improved high altitude capabilities and greater defensive maneuverability than the outmoded wooden biplane aircraft designs seen in the previous world war.
The 120mm Gun M1, or "120mm" as most called her (also the "Stratosphere Gun" as nicknamed by others), was the largest anti-aircraft gun constructed in the United States for the US Army during World War 2. She was first deployed in 1940 before the official American involvement in the conflict. Anti-aircraft guns were utilized to deter enemy aircraft from entering airspace by sending up heavy exploding shells at predetermined altitudes. Such defensive measures were fielded by all major powers participating in World War 2 and proved just as deadly a factor to airmen as enemy fighters would. The Germans found double success with their 88mm "FlaK" guns in the anti-tank role as well to which the "88s" fared quite favorably against enemy tank armor.
The M1 was conventionally mounted on a towing carriage making use of eight large road wheels set in pairs at each corner of the chassis. These were required to support her massive 62,000lbs (31 ton) transport weight. The M1 gun barrel could be set to fire vertically at targets up to an impressive 60,000 feet (18,000 meters) and, with a fairly trained crew, managing up to 10 rounds per minute. An experienced crew could loose up to 12 rounds per minute at targets while battletested crews were known to hit the 15 rounds per minute mark. The M1 made use of a 50 pound projectile that was produced with an internal 30-second time fuse delay. The weapon system required an operating crew of thirteen personnel made up of one officer, the gun commander, a gunnery sergeant, an ammunition sergeant (in charge of two ammunition handlers) and an additional seven-man support crew. Normally, a collection of four 120mm M1 systems would be fielded together to form a full anti-aircraft battery and provide a net of protection near vital installations.
For stability when firing the powerful 120mm cartridge, the M1 platform was designed with three hydraulically-powered leg "braces" each with base plates affixed to their ends. This three-point arrangement served as a recoil absorption system to counter the massive forces required in sending a 120mm shell upwards against the forces of gravity. These appendages could then be raised up and folded along the sides and front of the M1 system for ease of transport. When the braces were lowered to the ground completely, the wheels were raised and cleared from contact with the ground, relieving pressure to the air-filled tires. Along the rear of the gun carriage were two large folding platforms that allowed the gun crew a "stage" from which to operate the gun's elevation and traverse systems. These platforms were supported by smaller hydraulically-powered leg braces.
Accuracy in attacking incoming airborne targets was always an issue to any ground-based firing system. As such, the SCR-268 microwave radar system (later the SCR-584 series) was developed and integrated to locate enemy aircraft positions in the sky. The later SCR-584 would itself become the primary US Army anti-aircraft gun-finding system for the M1 series and, due to the technology of the time, was so large that it required its own transport caddy and operating crew. For illuminating enemy aircraft in dark hours of the day, an M1 gun battalion was also assigned with a high-powered aerial searchlight. With the target having been acquired by the radar system, the operating crew also needed a computer that could effectively calculate and triangulate a firing solution to the gun's firing crew - this inevitably became the M10 Auxiliary Predictor which allowed for the targeting of moving aircraft. Connected to this predictor was the M4 gun data computer which further aided the predictor and varied its recommendations based on the ammunition type being used and the overall distance to the target in question. Like the SCR-584 radar system, M4 gun data computer system was also physically large, noted as the size of a large, stand-up freezer unit. All told, the collective M1 package was cumbersome and heavy, making mobility an issue throughout her operational tenure.
The projectiles utilized with the M1 series were of two basic forms - shrapnel and High-Explosive (HE). Shrapnel projectiles sent a flurry of metal pieces around its detonation area while HE projectiles, as the name implies, simply exploded - both served to damage or outright destroy enemy aircraft and their crews. Since the inherent odds of directly contacting an enemy aircraft with the 120mm projectile remained slim despite the accessories designed to increase its accuracy, shells were fitted with a time fuse to allow for delayed airburst detonation. The shells themselves were not always accurate due to different burning rates of the powder within, particularly within the changing air pressures of the atmosphere. As such, different fuses were used to help reduce the issue. However, if the timer did not work as intended, the unexploded shells could have a detrimental effect on friendly forces on the ground instead. The vertical firing of the gun at a target could reach the stratosphere layer of the atmosphere and the M1 became the only such anti-aircraft gun in the war to showcase such a "reach". The gun was limited to the muzzle velocity being dependent on the ammunition and the environmental conditions at hand during firing meaning that both rain and winter-like conditions had their effects. With that said, however, the true ultimate limitation of the M1's maximum range was in the 30-second timer delay fuse of her projectiles.
Even though the M1 120mm AA gun was powerful and relatively accurate when connected to the best fire control computers of the day, she still maintained a major notable drawback in her design. She proved not very mobile, having to be towed by an M6 tractor that could only make 20 miles per hour on roads and only 5 miles per hour off road. As such, the gun proved impractical for tactical use in the Pacific Theater's island-hopping campaigns and, by the late war months in the European Theater where mobility was a major asset, the M1 system also suffered. As a result, most of these guns stayed within the borders of the United States proper, used to protect the American West Coast against expected Japanese attacks that never formulated. About fifteen M1 guns were sent to the Panama Canal Zone and several batteries were stationed in and around Allied London to aid in protecting British civilians against Hitler's V1 rocket revenge weapons. Despite its deployment in these active theaters of war, the M1 would never be fired against an enemy aircraft in the whole of the war. In 1944, the "4.7-inch M1" designation gave way to the more standard and identifiable "120mm Gun M1" naming convention recognized today.
After World War 2, the guns were brought back into operational service during the upcoming Korean War (1950-1953) with the Communist North's invasion of the democratic South. These M1s saw some success in their role but the day of the low-flying, piston-engined aircraft was coming to a close with the advent of jet-powered mounts such as the MiG-15 and F-86 Sabre. This, in effect, made anti-aircraft cannon-based systems such as the M1 obsolete for practical air defense. To counter the new aerial threats of the Soviet Union, a bevy of anti-aircraft guided missiles were soon developed for use in both air-to-air combat as well as in ground-launched, surface-to-air batteries. As with the anti-aircraft cannons before them, these missile launching systems could collectively be fielded to form a networked counter against any incoming aerial threat, be they bombers or fighters. Expectedly, the Soviet Union - and all other world powers of the time - followed suit with similar missile-based developments.
In all, only 550 120mm Gun M1 examples were produced.
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