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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