16"/50 caliber Mark 7 gun

The 16"/50 caliber Mark 7 – United States Naval Gun was the main armament of the Iowa-class battleships.

Description

A cutaway of a turret mounting 16-inch guns

Due to a lack of communication during design, the Bureau of Ordnance assumed the Iowa class would use the 16"/50 Mark 2 guns constructed for the 1920 South Dakota-class battleships. However, the Bureau of Construction and Repair assumed that the ships would carry a new, lighter, more compact 16"/50 and designed the ships with barbettes too small to accommodate a 16"/50 Mark 2 three-gun turret. The new 16"/50 Mark 7 was designed to resolve this conflict.

These guns were 50 calibers long—or 50 times their 406 mm bore diameter, which makes the barrels 20 m long, from breechface to muzzle. Each gun weighed about 108,000 kg without the breech, and 121,517 kg with the breech.^[1] They fired projectiles weighing from 850 to 1,200kg at a maximum speed of 820 m/s with a range of up to 39 km. At maximum range the projectile spent almost 1½ minutes in flight.^[1] Each turret required a crew of 79 men to operate.^[1] The turrets themselves cost US$1.4 million each, to which the cost of the guns had to be added.^[1]

The turrets (technically they are "gunhouses") were "three-gun", not "triple", because each barrel could be elevated and fired independently. The ships could fire any combination of their guns, including a broadside of all nine. The turret interiors were subdivided and so designed as to permit the independent loading, elevation and firing of each gun. Each turret was also installed with an optical range finder and ballistic analog computer. This permitted the turret's gun captain and crew to locally engage targets, should battle damage disrupt communication with the ship's primary or auxiliary fire control centers. Contrary to popular belief, the ships did not move sideways noticeably when a broadside was fired; this is simply an illusion. With the enormous mass of the vessel and the damping effect of the water around the hull the pressure wave generated by the gunfire was felt much more than the slight change in lateral velocity.^[2]

The guns could be elevated from −5 degrees to +45 degrees, moving at up to 12 degrees per second. The turrets could rotate about 300 degrees at about 4 degrees per second and could even be fired back beyond the beam, which is sometimes called "over the shoulder". Within each turret, a red stripe on the wall of the turret, just inches from the railing, marked the boundary of the gun's recoil, providing the crew of each gun turret with a visual reference for the minimum safe distance range.^[3]

Complementing the 16"/50 caliber Mark 7 gun was a fire control computer, in this case the Ford Instrument Company Mark 8 Range Keeper. This analog computer was used to direct the fire from the battleship's big guns, taking into account several factors such as the speed of the targeted ship, the time it takes for a projectile to travel, and air resistance to the shells fired at a target. At the time the Montana class was set to begin construction, the rangekeepers had gained the ability to use radar data to help target enemy ships and land-based targets. The results of this advance were telling: the rangekeeper was able to track and fire at targets at a greater range and with increased accuracy.^[4] This gave the US Navy a major advantage in World War II, as the Japanese did not develop radar or automated fire control to the level of the US Navy. During their brief reactivation in the 1980s, the Iowa-class Battleships paired their Mark 7 guns with the modern Mark 160 Fire Control System.^[5]

Mark 8 "Super-heavy" shell

The Mark 7 gun was originally intended to fire the relatively light 1,020 kg Mark 5 armor-piercing shell. However, the shell-handling system for these guns was redesigned to use the "super-heavy" 2,700-pound (1,200 kg) APCBC (Armor Piercing, Capped, Ballistic Capped) Mark 8 shell before any of the Iowa-class battleship's keels were laid down. The large-caliber guns were designed to fire two different 16 inch (406 mm) shells: an armor-piercing round for anti-ship and anti-structure work, and a high-explosive round designed for use against unarmored targets and shore bombardment.

The Mark 7 guns and the 2,700-pound projectiles were 25 percent lighter than the 46 cm/45 Type 94 naval guns of the Japanese Yamato-class battleships, but had nearly the same armor penetration ability.^[6]

The North Carolina and South Dakota classes could also fire the 2,700-pound Mark 8 shell, although with a shorter range, using the 16"/45 caliber Mark 6 gun. The Mark 6 gun was lighter than the Mark 7, which helped both battleship classes to conform to the limits of the Washington Naval Treaty.^[7]

The Mark 8 shells gave the North Carolina, South Dakota, and Iowa classes the second heaviest broadside of all battleship classes, even though the North Carolina and South Dakota ships were treaty battleships. Only the Yamato-class super-battleships could throw more weight.

The D839 propellant (smokeless powder) grain used for full charges issued for this gun was 2 inches long (5.08 cm), 1 inch in diameter (2.54 cm) and had seven perforations, each 0.060 inches in diameter (0.152 cm) with a web thickness range of 0.193 to 0.197 inches (0.490 to 0.500 cm) between the perforations and the grain diameter. A maximum charge consists of six silk bags–hence the term bag gun–each filled with 110 pounds of propellant.^[8]

Construction

Yard workers hoist one of nine 16"/50 Mark VII gun barrels aboard the USS Iowa during her construction in 1942.

The 16-inch/50 caliber Mark 7 guns of the forward turret of the battleship USS Wisconsin (BB-64) fires at enemy targets ashore on the Korean Peninsula on 30 January 1952 during the Korean War.

The built-up gun is constructed of liner, tube, jacket, three hoops, two locking rings, tube and liner locking ring, yoke ring and screw box liner. Some components were autofretted. Typical of United States naval weapons built in the 1940s, the bore was chromium-plated for longer barrel life. It uses a Welin breech block that opens downwards and is hydraulically operated. The screw box liner and breech plug are segmented with stepped screw threads arranged in fifteen sectors of 24 degrees each.

Service History

Off of Truk Atoll on 16 February 1944, Iowa and New Jersey engaged the Japanese destroyer Nowaki at a range of 32.6 km and straddled her, setting the record for the longest-ranged straddle in history and (from some reports) caused splinter damage and casualties to the crew from near misses. But Nowaki escaped due to the range and her speed. This action was part of Operation Hailstone which should be referenced for details, the only known surface engagements Iowa class battleships engaged in, during which they aided or completed the sinking of the IJN light cruiser Katori, the destroyer Maikaze, and the auxiliary cruiser Akagi Maru.

The gun was also used as the basis for Project HARP, an experimental gun-based space-launch research program.

See also

• List of naval guns
• List of World War II artillery
• 40 cm/45 Type 94 naval gun
• Armament of the Iowa class battleship
• List of the largest cannon by caliber
• 16"/45 caliber Mark 6 gun

Weapons of comparable role, performance and era

• 40.6 cm SK C/34 gun German equivalent

References

External links

Wikimedia Commons has media related to 16"/50 caliber Mark 7 gun.

• NavWeaps
• Video The 16"/50 Gun & Turret: US Navy Instructional Film
• USS Missouri rolling fire