The Mark 12 5"/38-caliber gun was a United States dual-purpose naval gun, but also installed in single-purpose mounts on a handful of ships. The 38-calibers long barrel was a mid-length compromise between the previous United States standard 5-inch/51 low-angle gun and 5-inch/25 anti-aircraft gun. The increased barrel length provided greatly improved performance in both anti-aircraft and anti-surface roles compared to the 5-inch/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 , commissioned in 1934, the first new destroyer design since the last Clemson was built in 1922. The base ring mount, which improved the effective rate of fire, entered service on , commissioned in 1936.
Among naval historians, the 5"/38 gun is considered the best intermediate-caliber, dual purpose naval gun of World War II, especially as it was usually under the control of the advanced Mark 37 Gun Fire Control System which provided accurate and timely firing against surface and air targets. Even this advanced system required nearly 1,000 rounds of ammunition expenditure per aircraft kill.Naval Weapons of WW2, Campbell, p106 However, the planes were normally brought down by shell fragments and not direct hits; barrage fire was used, with many guns firing in the air at the same time. This would result in large walls of shell fragments being put up to take out one or several planes or in anticipation of an unseen plane, this being justifiable as one plane was capable of significant destruction. The comparatively high rate of fire for a gun of its caliber earned it an enviable reputation, particularly as an anti-aircraft weapon, in which role it was commonly employed by United States Navy vessels. Base ring mounts with integral hoists had a nominal rate of fire of 15 rounds per minute per barrel; however, with a well-trained crew, 22 rounds per minute per barrel was possible for short periods. 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 (EFC) per barrel.Naval Weapons of WW2, Campbell, p139
The 5"/38 cal gun was mounted on a very 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, usually replacing 5"/25 guns that were fitted in the 1930s. 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. Millions of rounds of ammunition were produced for these guns, with over 720,000 rounds still remaining in Navy storage depots in the mid-1980s because of the large number of Reserve Fleet ships with 5"/38 cal guns on board.
Mark 12 5"/38 cal gun assembly
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 right 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 Mk 12 gun assembly weighs . The Mark 12 Gun Assembly was introduced in 1934, where it was first used in single pedestal mounts on the s, but by the time of World War II they had been installed in single and twin mounts on nearly every major warship and auxiliary in the US fleet.
Characteristics The major Mk12 Gun Assembly characteristics are:
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 propellant case is ejected, and the bore is cleared of debris with an air blast.
Hand loaded A projectile-man and a powder-man are stationed at each gun assembly. Their job is to move the round, consisting of a projectile and a propellant case, from the hoists to the rammer tray projecting from the gun's breech, and then start the ram cycle. The powder case is placed first and the projectile rests against the powder case.
Power rammed This gun used a electric-hydraulic power rammer that was designed to ram a , round (combined weight of projectile and propellant) into the chamber at any gun elevation in less than one second. The rammer's control box, hydraulic fluid tank and AC motor are bolted to the top of the slide (see the Gun Assembly picture above). 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" are confusing, look at this footnote.What this article calls the "rammer spade" is called the "power spade" in the gun assembly picture, and is parenthetically called the "shell guard" in the rammer tray illustration. This is probably the result of different Navy documents written by different people at different times. Ultimately, it became the "rammer spade" as shown in the rammer tray illustration. This is because it looked like a small, leather-covered garden spade—and it was what rammed the round into the chamber. Hand ramming was also possible in case of power failures with rate of fire approximately cut in half.
Vertical sliding-wedge breech block (see drawing) The breech block closes the chamber behind the propellant case. It also contains the firing pin assembly.
Hydraulic recoil Two hydraulic pistons in the housing absorb the major shock of recoil as the housing moves back inside the slide (see picture). They also buffer the end of counter-recoil for a soft return to battery.
Pneumatic counter-recoil At the end of recoil, the counter-recoil system moves the housing forward again until it is back "in battery," and holds it there at any gun elevation. A chamber in the housing is filled with compressed air. At the rear of this chamber is a cylindrical hole with a chevron packing. Projecting through the hole is a movable piston that abuts the back of the slide (see picture). The air pressure in the chamber attempts to push the piston out of the chamber, but the piston can't move because of the slide. Therefore, since the piston can't go backward, the air pressure in the chamber forces the housing forward. When the gun is in battery, the pressure in this chamber is about . During recoil, the pressure rises to about .
Barrel designation (5"/38 Caliber) 5" is the caliber of this gun. The term "caliber" used here is the artillery caliber definition, and its value is equal to the bore diameter as measured from land to land. Lands are the raised portions of the bore rifling, between the grooves. (note: artillery caliber and small arms caliber are not the same). 38 caliber means that the barrel from breech face to muzzle is 38 calibers in length. As this gun's caliber is 5 inches (127mm), its barrel length is 38 times 5 inches: .
Barrel description Radially expanded monobloc steel alloy. Weight: The bore is chrome plated from the origin of rifling (forward end of the chamber) to the muzzle. 45 groove rifling with a full uniform right hand twist in 30 Calibers (). Maximum bore pressure of a new gun firing an anti-aircraft common projectile: . Barrel connected to the housing with a bayonet joint that allowed for its replacement by destroyer tenders in the theater of operation, without dismantling the breech mechanism or other parts.
Ballistics Maximum horizontal range with a projectile is . In the anti-aircraft role, the gun has a ceiling of at 85 degrees elevation.
Armor penetration with special common shell 5.0" (127 mm) belt armor at 4.0" (102 mm) at 3.0" (76 mm) at 2.0" (51 mm) at 1.0" (25 mm) deck armor at with AA common shell: at
Range with AAC Mark 49 (792 mps) 10° 15° 20° 25° 30° 35° 40° 45° AA Ceiling
The new gun initial muzzle velocity is , and the gun life average initial muzzle velocity is about .
US ships during World War II carried only small quantities of special common ammunition, as the anti-aircraft common was considered more useful, even if it meant achieving much less armor penetration. Bursting charges were explosive D composition A, the special common had only 0.9-1.2 kg due to its thicker walls (AP).
Loading At the "LOAD" command, or if the mount is executing "RAPID" loading: (NOTE: Rapid loading means that, when the gun fires, the gun crew immediately reloads the gun without command until "CEASE FIRE" or "CHECK FIRE" is given.)
The Powder-Man: Verifies that the RAMMER SPADE is at the rear of the "Rammer Tray" (see picture) Slips the "Butterfly" primer protector off the base of the powder case that is sticking knee high out of powder hoist on the deck. Throws the protector out of the mount. Pulls the powder case out of the hoist, and lifts it into the rammer tray. Verifies that the case is back against the rammer spade. Clears his arms from the tray. The Projectile-Man: Verifies that the powder case is in the rammer tray. Pulls the projectile out of the waist high projectile hoist. Places projectile in front of the powder case in the tray. Clears his arms from the tray. Pulls down on the RAMMER CONTROL (see RAMMER CONTROL at the upper left corner of picture). Rammer Load Cycle: Pulling down on the Rammer Control lever opens hydraulic valves inside the Rammer Controller. The opened valves port high pressure hydraulic fluid to the "Hydraulic Ram Cylinder" in the Slide behind the tray. The Ram Cylinder is connected to the "Rammer Cross Head". (The white horizontal bar behind the Spade in the picture.) The Cross Head supports the Rammer Spade as it moves in the tray. When the cylinder is pressurized, the Cross Head and Spade are driven forward in the Tray. As the spade moves forward, it pushes the projectile and powder case into the chamber. The spade's time of travel, from the rear of the tray to the breech, is one second. As it approaches the breech, the Cross Head pushes through a spring latch that locks it there. At the same time, the Cross Head trips another lever connected to the Rammer Controller, and the Controller releases the hydraulic pressure to the Ram Cylinder. When the powder case clears the top of the breech block, the block automatically rises. The rising breech block displaces the rammer spade behind the powder case by pushing the spring-loaded spade up. This continues until the rammer spade is off the powder case, and the block has sealed the chamber. The gun is loaded and ready to fire.
Firing This gun can be fired either electrically or by percussion.
When the gun fires, the following automatic events happen in two seconds: During Recoil: The Rammer Crosshead is unlocked. The Rammer Control is pushed to the up position by a cam on top of the rearward moving Housing. With the Rammer Control up, the Rammer Controller ports high pressure hydraulic fluid to the retract side of the Rammer Cylinder, and the Rammer Crosshead and Spade drive back to the rear of the Rammer Tray. During Counter-recoil: The Firing Pin is cocked. The Breech Block is lowered. The spent Powder Case is ejected out of the Chamber, and back down the Rammer Tray. There it is caught by the gloved Hot Case Man, and thrown out of the mount. Just before the housing finishes returning to "In Battery", a valve is opened for a second, and a blast of compressed air is sent down the bore to clean it out. The gun is ready to be reloaded.
Gun mounts Types
There are four basic mount types:
Twin All sat on a base ring stand, and had an ammunition handling room, called the Upper Handling Room (as it was above the main magazine), below the mount. They all had: Horizontal periscopic sights with movable-prism sight setting. Two powered, fuze setting, projectile hoists.