The 8"/55 caliber gun (spoken "eight-inch-fifty-five-caliber") formed the main battery of United States Navy heavy cruisers and two early aircraft carriers. United States naval gun terminology indicates the gun barrel had an internal diameter of 8 inches (203 mm), and the barrel was 55 calibers long (barrel length is 8 inch × 55 = 440 inches or 36.6 feet or 11 meters).

Mark 9

These built-up guns weighed about 30 tons including a liner, tube, jacket, and five hoops. A down-swing Welin breech block was closed by compressed air from the gas ejector system. Loading with two silk bags each containing 45 lb of smokeless powder gave a 260 lb projectile a velocity of 2800 feet per second (853 m/s). Range was 18 miles 31860 yd at the maximum elevation of 41 degrees.

Mark 12

These simplified built-up guns eliminated hoops to reduce weight to 17 tons. The breech mechanism was similar and loading two silk bags each containing 43 pounds (20 kg) of smokeless powder gave a 335-pound (152 kg) projectile a velocity of 2500 feet per second (760 m/s). Each gun could fire about four rounds per minute. Maximum range was 30050 yd at the maximum elevation of 41 degrees.

Mark 14

These guns were similar to Mark 9, with the same shell weight and maximum range, with a smaller chamber and rifling twist increased from 1 in 35 to 1 in 25 in a chromium-plated bore.

Mark 15

These guns were similar to Mark 12, with the same shell weight and maximum range, with the smaller chamber of the Mark 14 gun. Useful life expectancy was 715 effective full charges (EFC) per liner.

Mark 16

These self-loading guns with lined monobloc construction and vertical sliding breech blocks weighed about 20 tons. Semi-fixed ammunition (projectile and powder case handled separately) with 78 pounds (35 kg) of smokeless powder gave a 335-pound (152 kg) projectile a velocity of 2500 feet per second (760 m/s). Each gun could fire about ten rounds per minute. Useful life expectancy was 780 Effective Full Charges per liner. Range was 17 miles (27 kilometers) at the maximum elevation of 41 degrees. This gun was modified for the experimental Major Caliber Lightweight Gun.

Coast defense use

The eight twin turrets of USS Lexington (CV-2) and USS Saratoga (CV-3) were removed in early 1942 during refits at Pearl Harbor. The turrets were turned over to the United States Army Coast Artillery Corps and remounted as coastal artillery on Oahu. Four two-turret batteries were established at Salt Lake near Aliamanu Crater (Battery Salt Lake, later Battery Burgess), Wiliwilinui Ridge Military Reservation (Battery Wilridge, later Battery Kirkpatrick), Opaeula Military Reservation (Battery Opaeula, later Battery Riggs), and Brodie Camp Military Reservation (Battery Brodie, later Battery George Ricker). After the war, all of the guns and turrets were scrapped in 1948, along with almost all other US coast artillery. One of USS Louisville (CA-28)'s main battery 8 inch 55 caliber gun turrets (Turret No. 2) damaged in a kamikaze attack on January 5, 1945, was removed and taken to the Nevada Test Site and converted into a rotating radiation detector, to collect data on nuclear tests.

Ships mounting 8"/55 caliber guns

Weapons of comparable role, performance and era