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.
Early low-angle guns
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 guns of the World Wars
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. The gun was also used on specialist destroyer conversions; the "AVD" seaplane tender conversions received two guns; the "APD" high-speed transports, "DM" minelayers, and "DMS" minesweeper conversions received three guns, and those retaining destroyer classification received six.
These dual-purpose guns were "quick-firing", meaning that they used fixed ammunition, with powder case and projectile permanently attached, and handled as a single unit weighing 34 pounds (as opposed to older guns and/or heavier guns, in which the shell and powder are handled and loaded separately, which reduces the weight of each handled component, but slows the loading process). The shells alone weighed about 13 pounds including an explosive bursting charge of 0.81 pounds for anti-aircraft (AA) rounds or 1.27 pounds for High Capacity (HC) rounds, the remainder of the weight being the steel casing. Maximum range was 14,600 yards at 45 degrees elevation and ceiling was at 85 degrees elevation. Useful life expectancy was 4300 effective full charges (EFC) per barrel.
This is not to be confused with the "rapid-fire" of later gun mounts that used an autoloader mechanism to insert the fixed QF ammunition into the breech. This in turn is not to be confused to a fully automatic gun. The autoloader was still manually filled with shells.
Submarine deck guns
The 3"/50 caliber gun Marks 17 and 18 was first used as a submarine deck gun on R-class submarines launched in 1918–1919. At the time it was an improvement on the earlier 3"/23 caliber gun. After using larger guns on many other submarines, the 3"/50 caliber gun Mark 21 was specified as the standard deck gun on the Porpoise- through s launched in 1935–1942. The small gun was chosen to remove the temptation to engage enemy escort vessels on the surface. The gun was initially mounted aft of the conning tower to reduce submerged drag, but early in World War II it was shifted to a forward position at the commanding officer's option. Wartime experience showed that larger guns were needed. This need was initially met by transferring 4"/50 caliber guns from S-class submarines as they were shifted from combat to training roles beginning in late 1942. Later, the 5"/25 caliber gun, initially removed from battleships sunk or damaged in the attack on Pearl Harbor and later manufactured in a submarine version, became standard.