JR Potts, AUS 173d AB (Updated: 6/23/2015):
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.