The 3-inch antiaircraft gun was the respected grandfather of all World War II antiaircraft artillery. Shortly after America's entry into the First World War, the caliber of Army antiaircraft guns was set at three inches to take advantage of existing standard cartridges. Both fixed mount guns (M1917) and mobile guns (M1918) were soon developed. The 3-inch gun designs continued to be refined during the interwar period with appearance of the more robust M1 and M3 mobile guns and the fixed mount M2 and M4 pieces. By 1930, the M3 was settling in as the standard mobile gun. This allowed a decade of training with the improved weapon before the onset of World War II. Therefore, veteran Coast Artillery officers and men were thoroughly familiar with their 3-inch antiaircraft guns by the time Pearl Harbor was attacked and America found herself thrust into the global conflict.
Even though the 3-inch gun was a familiar face and a fine performer, many were concerned that the old man would not be able to keep pace with modern bombers, which were now flying faster and at greater altitudes. Consequently, the Army ordered development of a heavier antiaircraft gun capable of dealing with the latest aircraft. This new 90mm gun was standardized in the spring of 1940 and would soon end the two-decade reign of the 3-incher as king of AA.
As the 90mm went into production, it slowly began to supplant the 3-inch guns in pre-war Coast Artillery antiaircraft organizations. However, it was necessary to continue to employ the aging 3-inchers as the national emergency of the Second World War gripped the United States and challenged the resources of the nation and its Armed Forces. Many antiaircraft units remained armed with 3-inch pieces throughout the early stages of the war, especially those organizations already deployed abroad prior to the attack on Pearl Harbor. As if they sensed their impending obsolescence, the older guns nobly rose to the task and stood tall in accordance with their time-tested tradition.
The gun section of a World War II period 3-inch firing section, as defined in 1942, consisted of a gun squad and an ammunition squad. The gun squad was led by a sergeant called the gun commander, who also served as chief of the entire gun section. He was assisted in the gun squad by the gunner and fuze range setter, both corporals. The squad was rounded out by an elevation setter, azimuth setter, and six other enlisted men serving as cannoneers - a total of eleven men. This squad manned the 3-inch gun proper.
The ammunition squad was commanded by the chief of ammunition, a corporal. Under him were nine cannoneers in a mobile unit or six cannoneers in a semimobile unit. The squad was responsible for the supply of ammunition to the gun, the cleaning and disposition of spent cases, and the safe handling of ammunition and related supplies.
Even after they were superseded by the 90mm guns, the Army found a use for their venerable 3-inchers. Since factories were already tooled to manufacture 3-inch gun barrels, and replacement parts for the fading old antiaircraft gun were on hand, the Army used these barrels to develop a new hybrid anti-tank artillery piece - the M5.
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