16"/50 caliber Mark 7 gun

The 16"/50 caliber Mark 7 – United States Naval Gun is the main armament of the s.

Description
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 triple turret. The new 16"/50 Mark 7 was designed to resolve this conflict.

These guns were 66.6 feet (20 m) long—50 times their 16 in bore, or 50 calibers, from breechface to muzzle. Each gun weighed about 239,000 pounds (108,000 kg) without the breech, or 267,900 pounds (121,517 kg) with the breech. They fired projectiles weighing from 1,900 to 2,700 pounds (850 to 1,200 kg) at a maximum speed of 2690 ft/s with a range of up to 24 miles (39 km). At maximum range the projectile spent almost 1½ minutes in flight. Each turret required a crew of 94 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 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. Considering the large mass of the ship, compared to the mass of the projectiles, the ships barely moved sideways at all, even when a full broadside was fired. With 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". 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.

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 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. 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.

Mark 8 "Super-heavy" shell
The Mark 7 gun was originally intended to fire the relatively light 2240 lb Mark 5 armor-piercing shell. However, the shell-handling system for these guns was redesigned to use the "super-heavy" 2700 lb APC (Armor Piercing, Capped) Mark 8 shell before any of the s were laid down. The large caliber guns were designed to fire two different 16 in (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 s, but had nearly the same armor penetration ability.

The and  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 propellant consists of small cylindrical grains of smokeless powder with an extremely high burning rate. A maximum charge consists of six silk bags, each filled with 110 pounds of propellant.

Construction
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.