16-inch/50-caliber Mark 7 gun

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 .
Description.
Due to a lack of communication during design, the Bureau of Ordnance assumed the "Iowa" class would use the /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-in/50 and designed the ships with barbettes too small to accommodate a 16-in/50 Mark 2 three-gun turret. The new 16-in/50 Mark 7 was designed to resolve this conflict.
These guns were 50 calibers long—or 50 times their bore diameter, which makes the barrels long, from chamber to muzzle. Each gun weighed about without the breech, and with the breech. They fired projectiles weighing from at a maximum speed of with a range of up to . 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 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 Mark 5 armor-piercing shell. However, the shell-handling system for these guns was redesigned to use the "super-heavy" APCBC (Armor Piercing, Capped, Ballistic Capped) Mark 8 shell before any of the '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 s, 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 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 D839 propellant (smokeless powder) grain used for full charges issued for this gun was long, in diameter and had seven perforations, each in diameter with a web thickness range of between the perforations and the grain diameter. A maximum charge consisted of six silk bags–hence the term bag gun–each filled with of propellant.
Construction.
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 at a range of 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.
Ballistic Characteristics.
2,500 ft/s (762 m/s) muzzle velocity with 2,700 lb (1,225 kg) AP shell 
Armor Penetration.
2,500 ft/s (762 m/s) muzzle velocity with 2,700 lb (1,225 kg) AP shell 