The Mark 12 5/38 caliber gun was a US naval gun. The gun was installed into Single Purpose and Dual Purpose mounts used primarily by the US Navy, the 38 caliber barrel was a mid-length compromise between the previous United States standard 5/51 low-angle gun and 5/25 anti-aircraft gun. The increased barrel length provided greatly improved performance in both anti-aircraft and anti-surface roles compared to the 5/25 gun, however, except for the barrel length and the use of semi-fixed ammunition, the 5/38 gun was derived from the 5/25 gun. Both weapons had power ramming, which enabled rapid fire at high angles against aircraft, the 5/38 entered service on USS Farragut, commissioned in 1934. The base ring mount, which improved the rate of fire, entered service on USS Gridley. Even this advanced system required nearly 100 rounds of ammunition expenditure per aircraft kill, however, the planes were normally killed by shell fragments and not direct hits, barrage fire was used, with many guns firing in the air at the same time. Base ring mounts with integral hoists had a rate of fire of 15 rounds per minute per barrel, however. On pedestal and other mounts lacking integral hoists,12 to 15 rounds per minute was the rate of fire, useful life expectancy was 4600 effective full charges per barrel. The 5/38 cal gun was mounted on a large number of US Navy ships in the World War II era. It was backfitted to many of the World War I-era battleships during their wartime refits and it has left active US Navy service, but it is still on mothballed ships of the United States Navy reserve fleets. It is also used by a number of nations who bought or were given US Navy surplus ships, each mount carries one or two Mk 12 5/38cal Gun Assemblies. The gun assembly shown is used in single mounts, and it is the gun in twin mounts. It is loaded from the left side, the left gun in twin mounts is the mirror image of the right gun, and it is loaded from the right side. The Mk12 gun assembly weighs 3,990 lb, the major Mk12 Gun Assembly characteristics are,158 Semi-automatic During recoil, some of the recoil energy is stored in the counter-recoil system. That stored energy is used during counter-recoil to prepare the gun for the next round, the firing pin is cocked, the breech is opened, the spent powder case is ejected, and the bore is air cleaned. Hand loaded A Projectile-Man and a Powder-Man are stationed at each gun assembly and their job is to move the round, consisting of a projectile and a powder case, from the hoists to the rammer tray, and then start the ram cycle. The hydraulically driven Rammer Spade, called the Power Spade in that picture, is at the back of the Rammer Tray, if the multiple names of the Spade is confusing, look at this footnote. Vertical sliding-wedge breech block The breech block closes the chamber behind the powder case and it also holds the firing pin assembly
Image: Fletcher class destroyer detail
Mk 12 gun assembly
Vertical Sliding Wedge Breech Block.
Drawing of the recoil and counter-recoil systems. The arrow shows the motion of the housing in the slide during recoil.