120 mm M1 gun

The 120 mm Gun M1 was the United States Army's standard super-heavy anti-aircraft gun, complementing the smaller and more mobile M2 90 mm gun in service. Its maximum altitude was about 60000 ft, which garnered it the nickname the stratosphere gun. The 120 served primarily in static defensive roles, although it had been designed to be mobile. One battalion, the 513th AAA Gun Battalion, did make it to the Philippines in February 1945, although they never fired their guns in anger. There is also speculation that some 120s arrived in England in early 1945 to provide air defense for London, although no concrete evidence of this has been documented. The M1 gun was used in air defense, from 1944 through the early cold war period when the system was gradually phased out with active duty units,beginning in 1954 when the Nike Ajax surface to air missile gave air defenders a significantly greater long range capability.Cold war Army ARAACOM planning against long range Soviet Tu-4 bomber attack included 44 active duty and 22 National guard battalions divided into seven brigades and 20 anti-aircraft artillery groups as well as two Canadian antiaircraft battalions for the joint defense of the Sault Ste Marie Canal,equipped with the M1 120mm gun,guided by long range Lashup AN/CPS-5 radars for the protection of Nuclear production facilities,major Industrial centers,Strategic Air force bases,and select major population centers.the construction of the Pinetree, Mid-Canada and DEW line, Early Warning Radars which were all completed by 1957, along with Large scale deployment of U.S. and Canadian jet Interceptors Equipped with the AIR-2 Genie Nuclear Air to Air missile,and introduction of the First Soviet ICBM the R7 in 1957,all rendered Heavy AA guns obsolete. remaining National Guard heavy gun units converted to Nike or had been deactivated by January 1960.

History
The Army had originally attempted to make a 120 mm design just after the end of World War I, with a prototype being presented in 1924. The system was considered far too heavy and expensive to be useful, and the project slowed down, although it was never cancelled outright.

In 1938, the Army started studying its needs for newer AAA systems, and decided to order new systems for both the heavy and super-heavy role. The former was filled by the new M1 90 mm gun, which replaced the earlier M3 3-inch gun then in use, while work on the 120 mm gun was dusted off and mated to a new eight-wheel carriage to produce the 4.7-inch M1 when it was accepted in 1940.

Like the 90 mm, the M1 was typically operated in a battery of four guns, initially with an associated searchlight and SCR-268 radar, and later with the much-improved SCR-584 radar, an M10 Director (Military) and an M4 Gun Data Computer that automatically laid the guns as well. The M6 Tractor was used as the prime mover.