1.1-inch/75-caliber gun

1.1" / 75 Caliber Gun
Quadruple-mount 1.1-inch (28 mm) anti-aircraft cannon aboard the battleship USS Pennsylvania during World War II
Type	Anti-aircraft gun
Place of origin	United States
Service history
Used by	United States Navy
Wars	World War II
Production history
Produced	1938–1942[1][Note 1]
No. built	~1000[1]
Specifications
Mass	10,500 lb (4,800 kg)[1]
Length	119.6 in (3.04 m)
Barrel length	82.5 in (2.10 m) bore (75 calibers)
Crew	15
Shell	28 x 199mmSR 0.917 lb (0.416 kg) contact HE.
Caliber	1.1-inch (28 mm)[2]
Recoil	3.25-inch (83 mm)
Elevation	-15 to 110 degrees[1]
Traverse	360 degrees
Rate of fire	150 rounds per minute
Muzzle velocity	2,700 ft/s (820 m/s)[1]
Maximum firing range	7,000 yd (6,400 m)[1]

The 1.1"/75 caliber gun was an American anti-aircraft weapon of World War II, used by the United States Navy.^[3] The name means that it had a bore diameter of 1.1 in (28 mm) and barrel caliber of 75 (1.1 inches × 75 = 82.5 in (2.1 m)). The gun was designed to replace the M2 Browning and four barrels were required to duplicate the rate of fire.

The first shipboard installation, in 1939, was nicknamed the Chicago Piano.^[1]

By 1941, these guns had been mounted on destroyers, cruisers, battleships, aircraft carriers, and some auxiliary ships. Nearly a thousand guns had been produced before production shifted to more reliable shipboard anti-aircraft machine guns in 1942.^[1] Quantities were minimal; one mount for a destroyer, two mounts for pre-1930s battleships, and four mounts for North Carolina–class and newer battleships. On at least some ships they were director-controlled.

The gun was very unpopular with its crews; it was said that due to its tendency to jam, the only way to fire one was to position a gunner's mate on his back underneath the mount, equipped with an assortment of wrenches and hammers to clear them.^[4] It was replaced by the 20 mm (0.79 in) Oerlikon cannon or the 40 mm (1.6 in) Bofors gun whenever possible, but served until the end of the war on some ships. A twin Bofors gun was about the same weight, and was a much more powerful gun. The air-cooled Oerlikon had similar effective range and rate of fire with considerably less weight. The Oerlikon could not sustain fire for as long as the water-cooled 1.1–inch, but six Oerlikons could be installed for the weight of a single 1.1–inch quad mount.^[1]

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