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 and barrel length of 75 × 1.1 inches = , 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: During the early part of World War II, they were deployed on most U.S. Navy warships of destroyer and larger size. The guns were sometimes referred to as "Chicago Pianos."