The 1.1"/75 caliber gun was an American anti-aircraft weapon of World War II, used by the United States Navy. 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 quickly nicknamed the Chicago Piano, possibly because it was the size of a baby grand piano and looked a little like a baby grand without its lid. The name also references the "tommy guns" used by gangsters in Chicago. (Although the reminiscences of the sailors who served with these guns all agree on the nickname, no one seems to know where it originated.)