1.1"/75 caliber gun

The 1.1"/75 caliber gun was an American anti-aircraft weapon of World War II. The name means that it had a bore diameter of 1.1 in and barrel length of 75 × 1.1 inches = 82.5 in, using the "caliber" nomenclature common to large artillery pieces.

It was developed when the U.S. Navy Bureau of Ordnance (BuOrd) decided that the .50 caliber machine gun was not adequate for future AA duties:


 * "In recognition of the growing necessity for antiaircraft fire, the more or less continuous Bureau experimentation with double-purpose guns during the 1920s finally culminated in the early 1930s in the development of the 5"/38 DP gun which fulfilled its mission throughout the war with very little criticism. While the longer range antiaircraft gun field was taken care of, except for insufficient numbers, the situation was far from satisfactory in the short range category. Neither the .50 caliber machine gun, effective enough in plane-to-plane fire at pointblank range, nor the 1.1" which the Bureau developed in quadruple mounts in the 1930s, were competent to meet the menace of the Second World War plane. The 1.1" was too heavy to serve as a "last-ditch" free mount and too light to span the gap between the small machine guns and the 5-inch guns, even had all its "bugs" been eliminated. The lack of adequate short range antiaircraft guns together with insufficient quantities of the best guns then available created a situation which by 1940 could hardly be termed anything but critical."



During the early part of World War II, they were deployed on most U.S. Navy warships of destroyer and larger size.

Before the Japanese struck Pearl Harbor on December 7, five 1.1 caliber quad mounts had been sent to Cavite Navy Yard in Manila Bay, Philippines for fitting to the cruiser USS Houston (CA-30) of the Asiatic Fleet. Four were mounted on the Houston and the fifth was a spare. To the surprise of most at Cavite, the one spare left on the dock survived the Japanese bombing. Since the mount was too heavy for the few harbor patrol vessels still stationed in Manila Bay, the fifth spare mount was put on a barge along with 25,000 rounds of 1.1 ammunition and taken to Corregidor and donated to the US Army. There is no further record of what happened with that one 1.1 caliber mount sent to Corregidor.

Some online articles referring to this "donated to the Army" mount exist. One states the guns were installed in a special concrete mount and used successfully against Japanese airplanes until destroyed by gunfire.

Due to teething problems that were never rectified, the gun was very unpopular with its crews. It was replaced with the Bofors 40 mm gun whenever possible, but served until the end of the war on some ships.

The guns were sometimes referred to as "Chicago Pianos."