16-inch/50-caliber Mark 7 gun

16"/50 caliber Mark 7 gun
The Iowa-class battleship USS Iowa fires a full broadside of her 16"/50 Mark 7 guns.
Type	Naval gun
Place of origin	United States
Service history
In service	1943–1992
Used by	U.S. Navy
Wars	World War II Korean War Vietnam War Lebanese Civil War Gulf War
Production history
Designed	1939
Specifications
Mass	267,904 lb (121,519 kg) (including breech)
Length	816 in (68.0 ft; 20.7 m)
Barrel length	800 in (67 ft; 20 m) (50 calibers)
Shell	AP Mark 8: 2,700 lb (1,225 kg) HC Mark 13: 1,900 lb (862 kg) Nuclear Mark 23 (W23): 1,900 lb (862 kg)
Caliber	16 in (406 mm)
Muzzle velocity	AP: 2,500 ft/s (762 m/s) HC & Nuclear: 2,690 ft/s (820 m/s)

The 16"/50 caliber Mark 7 – United States Naval Gun is the main armament of the Iowa-class battleships and was the planned main armament of the cancelled Montana-class battleship.

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-inch (406 mm)/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 compact 16-in/50 turret and designed the ships with barbettes too small to accommodate the 16-in/50 Mark 2 three-gun turret that the Bureau of Ordnance was actually working on. The new lightweight 16-in/50 Mark 7 was designed to resolve this conflict.

These guns were 50 calibers long—or 50 times their 16-inch (406 mm) bore diameter, which makes the barrels 66.7 ft (20.3 m) long, from chamber to muzzle. Each gun weighed about 239,000 lb (108,000 kg) without the breech, and 267,900 lb (121,500 kg) with the breech. They fired projectiles weighing from 1,900 to 2,700 lb (860 to 1,220 kg) at a maximum speed of 2,690 feet per second (820 m/s) with a range of up to 24 mi (39 km). At maximum range the projectile spent almost 1½ minutes in flight. Each turret required a crew of 79 men to operate. The turrets themselves cost US$1.4 million each, to which the cost of the guns had to be added.

The turrets 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 designed to permit the independent loading, elevation and firing of each gun. Each turret was also installed with an optical range finder, ballistic analog computer, and a switchboard. The rangefinder and ballistic computer 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. The firing switchboard allowed any remaining fire control computer to send data to or control the firing computers of other turrets in the event of battle damage to the primary and secondary artillery plotting rooms.

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.

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". A red stripe on the wall of each turret, just inches from the railing, marked the limit of the gun's recoil as a safety warning to the turret's crew.

Complementing the 16-in/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 projectile's travel time, and air resistance. 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 greater range and with increased accuracy, day or night. This gave the US Navy a major advantage in the later half of WWII, as the Japanese did not develop radar or automated fire control to the level of the US Navy.

During their reactivation in the 1980s, the modern Mark 160 Fire Control System was used to guide the fire of the Iowa-class Battleship Mark 7 guns.

Mark 8 "Super-heavy" shell

The Mark 7 gun was originally intended to fire the relatively light 2,240-pound (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 19.2 percent lighter than the 46 cm/45 Type 94 naval guns of the Japanese Yamato-class battleships, but had better armor penetration ability at long range. (Sectional density = 6.9 grams per square millimeter for the IJN shell and 7.43 grams per square millimetrer in the USN gun.)

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.

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 in (51 mm) long, 1 in (25 mm) in diameter and had seven perforations, each 0.060 in (1.5 mm) in diameter with a web thickness range of 0.193 to 0.197 in (4.9 to 5.0 mm) between the perforations and the grain diameter. A maximum charge consisted of six silk bags–hence the term bag gun–each filled with 110 lb (50 kg) of propellant.

16"/50 gun projectile with six propellant bags on display aboard the USS Iowa (BB-61)

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) fire at enemy targets ashore on the Korean Peninsula on 30 January 1952 during the Korean War.

Employees working with the automatic 16-inch powder stacking machine at Naval Ammunition Depot Hingham, Mass. during World War II.

The Mark 7 gun was a built-up gun and was 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 had a Welin breech block that opened downwards and was hydraulically operated. The screw box liner and breech plug was 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 35,700 yards (32.6 km) and straddled her, setting the record for the longest-ranged straddle in history. Some reports indicated the near misses caused splinter damage and casualties to the crew. Nowaki was able to escape due to the range and her speed. The action against Nowaki was part of Operation Hailstone. This is the only surface engagement that Iowa-class battleships are known to have engaged in. In the action they participated in 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.

Gun characteristics

Designation	16-in/50 caliber (406 mm × 20.3 m) Mark 7
Ship class(es) used on	Iowa (BB-61) and Montana (BB-67) classes
Date of design	1939
Date entered service	1943
Gun weight	267,904 lb (121,519 kg) (including breech) 239,156 lb (108,479 kg) (without breech)
Gun length oa	816 in (68.0 ft; 20.7 m) (breech face to muzzle)
Bore length	800 in (67 ft; 20 m)
Rifling length	682.86 in (56.905 ft; 17.345 m)
Grooves	(96)0.150 in (3.8 mm) deep
Lands	N/A
Twist	Uniform RH 1 in 25
Chamber volume	27,000 cu in (440,000 cm3)
Rate of fire	2 rounds per minute
Note: The primer cartridge could be fired electrically or percussion fired.
Range	41,622 yd (38.059 km; 20.550 nmi) with nominal 660 lb (300 kg) powder charge
Muzzle velocity	2,690 ft/s (820 m/s) with 1,900 lb (860 kg) HC (High Capacity) shell and 2,500 ft/s (760 m/s) with 2,700 lb (1,200 kg) AP (Armor Piercing) shell
Muzzle energy	290 MJ (81 kWh) for the light projectile and 355 MJ (99 kWh) for the heavy projectile.

See also

• List of naval guns
• List of World War II artillery
• 46 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

External links

• NavWeaps
• Video The 16"/50 Gun & Turret: US Navy Instructional Film
• USS Missouri rolling fire
• 16in Barrels: Construction and Maintenance Battleship New Jersey Museum
• Romano, Arthur R. (March 1987). "Reactivation Of 16-Inch Three Gun Turrets In The Battleship". 24th Annual Technical Symposium. Department of the Navy. https://drive.google.com/file/d/1Tl0lqFUTidus6_GEDj8wguhtbum0rVX4/view.