The 3"/50 caliber gun (spoken "three-inch fifty-caliber") in United States naval gun terminology indicates the gun fired a projectile in diameter, and the barrel was 50 calibers long (barrel length is 3 in × 50 = ). Different guns (identified by Mark numbers) of this caliber were used by the U.S. Navy and U.S. Coast Guard from 1900 through to 1990 on a variety of combatant and transport ship classes.
The gun is still in use with the Spanish Navy on Serviola-class patrol boats.
The US Navy's first 3"/50 caliber gun (Mark 2) was an early model with a projectile velocity of per second. Low-angle (single-purpose/non-anti-aircraft) mountings for this gun had a range of 7000 yards at the maximum elevation of 15 degrees. The gun entered service around 1900 with the s, and was also fitted to s. By World War II these guns were found only on a few Coast Guard cutters and Defensively Equipped Merchant Ships.
Low-angle 3"/50 caliber guns (Marks 3, 5, 6, and 19) were originally mounted on ships built from the early 1900s through the early 1920s and were carried by submarines, auxiliaries, and merchant ships during the Second World War. These guns fired the same ammunition used by the following dual-purpose Marks, but with range limited by the maximum elevation of the mounting. These were built-up guns with a tube, partial-length jacket, hoop and vertical sliding breech block.
Dual-purpose 3"/50 caliber guns (Marks 10, 17, 18, and 20) first entered service in 1915 as a refit to , and were subsequently mounted on many types of ships as the need for anti-aircraft protection was recognized. During World War II, they were the primary gun armament on destroyer escorts, patrol frigates, submarine chasers, minesweepers, some fleet submarines, and other auxiliary vessels, and were used as a secondary dual-purpose battery on some other types of ships, including some older battleships. They also replaced the original low-angle 4"/50 caliber guns (Mark 9) on "flush-deck" and s to provide better anti-aircraft protection.