16"/45 caliber Mark 6 gun

The 16"/45 caliber Mark 6 gun was a naval gun designed in 1936 by the United States Navy for their Treaty battleships. It was first introduced in 1941 aboard their s, replacing the originally intended 14"/50 caliber Mark B guns and was also used for the follow-up South Dakota class. These battleships carried nine guns in three triple turrets. The gun was an improvement to the 16"/45 caliber guns used aboard the, and the predecessor to the 16"/50 caliber Mark 7 gun used aboard the.

Description
The 16 in/45 were improved versions of the Mark 5 guns mounted on the s, with their limit of a 2240 lb shell with a maximum 35000 yd range at their turret limit of 30 degree elevation. A major alteration from the older guns was the Mark 6's ability to fire a new 2700 lb armor piercing (AP) shell developed by the Bureau of Ordnance. At full charge with a brand-new gun, the heavy shell would be expelled at a muzzle velocity of 2,300 feet per second (701 m/s); at a reduced charge, the same shell would be fired at 1,800 f/s (549 m/s).

Barrel life—the approximate number of rounds a gun could fire before needing to be relined or replaced—was 395 shells when using AP, increasing to 2,860 for practice rounds. By comparison, the 12"/50 caliber Mark 8 gun of the large cruisers had a barrel life of 344 shots, while the 16"/50 caliber Mark 7 gun fitted in the s had a barrel life of 290 rounds.

Turning at 4 degrees a second, each turret could train to 150 degrees on either side of the ship. The guns could be elevated to a maximum inclination of 45 degrees; turrets one and three could depress to −2 degrees, but due to its superfiring position, the guns on turret two could only depress to 0 degrees.

Each gun was 736 in long overall which is 45 bore diameters hence the 16"/45 caliber; its bore and rifling length were 720 in and 616.9 in, respectively. Maximum range with the heavy AP shell was obtained at an inclination of 45 degrees: 36900 yd. At the same elevation a lighter 1900 lb high capacity (HC) shell would travel 40180 yd. The guns weighed 192,310 lb (87,230 kg; 86 long tons) not including the breech; the turrets weighed slightly over 3,100,000 lb (1,410,000 kg; 1400 long tons).

When firing the same shell, the 16 in/45 Mark 6 had a slight advantage over the 16 in/50 Mark 7 when hitting deck armor—a shell from a 45 cal gun would be slower, meaning that it would have a steeper trajectory as it descended. At 35000 yd, a shell from a 45 cal would strike a ship at an angle of 45.2 degrees, as opposed to 36 degrees with the 50 cal.



Shell
The Mark 6 and 7 guns were originally intended to fire the relatively light 2240 lb (1.00 long ton) 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 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 8 shells gave the North Carolina, South Dakota, and Iowa classes the second heaviest broadside of all battleship classes, despite the fact that the North Carolina and South Dakota ships were treaty battleships. Only the super dreadnoughts could throw more weight. The Mark 6's disadvantage relative to other contemporary battleship classes was its comparatively shorter range.

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 (50 kg) of propellant.

Service history
The Mark 6 16 inch gun holds several distinctions relating to the United States' World War II combat history.

In the first instance, as the primary armament of USS Washington (BB-56) these guns were employed against the Imperial Japanese Navy's JAPANESE BATTLESHIP Kirishima during the Naval battle of Guadalcanal; this has been cited by historians as the only instance in World War II in which one American battleship actually sank an enemy battleship. (While there was a battleship versus battleship engagement at Leyte Gulf, torpedoes rather than gunfire were largely regarded as being responsible for sinking the enemy battleships.) The Washinton had the aid of a fire control computer&mdash;in this case the Ford Instrument Company Mark 8 Range Keeper analog computer used to direct the fire from the battleship's 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. This gave the US Navy a major advantage in the Pacific War, as the Japanese did not develop radar or automated fire control to a comparable level. Washington was able to track and fire at targets at a greater range and with increased accuracy, as was demonstrated in November 1942 when she engaged Kirishima at a range of 8400 yd at night. Using her nine 16"/45 caliber Mark 6 guns Washington fired 75 rounds of 16" AP shells and scored an incredible twenty heavy caliber hits that critically damaged the Kirishima, which eventually sunk. During the same battle, South Dakota also fired off several salvos from her 16"/45 guns before she had to withdraw due to a faulty circuit breaker.

In the second instance, the battleship USS Massachusetts (BB-59) employed these 16"/45 caliber guns as her primary armament, and she is believed to have to fired the United States' first and last 16 inch shells of World War II; the first use occurring 8 November 1942 during the Naval Battle of Casablanca (shortly before the Naval battle of Guadalcanal), the last being 9 August 1945 off the coast of Hamamatsu, Japan. Furthermore, at Casablanca, this was the only time in the European theater that a fast battleship of the US Navy fired her guns in anger.

Successor
The next US Navy battleship class, the, did not fall under Treaty weight restrictions and allowed for additional displacement. However in their original design, the General Board was incredulous that a tonnage increase of 10000 LT would only allow the addition of 6 knots over the South Dakotas. Rather than retaining the 16"/45 caliber Mark 6 gun used in the South Dakotas, they ordered that future studies would have to include the more powerful (but heavier) 16"/50 caliber Mark 2 guns left over from the canceled s and battleships of the early 1920s. It also allowed the draft of the ships to be increased, meaning that the ships could be shortened (lowering weight) and the power reduced (since a narrower beam reduces drag).

The Mark 2 50-caliber gun weighed some 400 LT more than the Mark 6 45 caliber did; the barbette size also had to be increased so the total weight gain was about 2000 LT, putting the ship at a total of 46551 LT—well over the 45,000 long ton limit. An apparent savior appeared in a Bureau of Ordnance preliminary design for a turret that could carry the 50 caliber guns in a smaller barbette. This breakthrough was shown to the General Board as part of a series of designs on 2 June 1938. Nonetheless, the Mark 7 gun still weighed about 239,000 pounds (108 000 kg) without the breech, or 267,900 pounds with the breech, considerably heavier than the Mark 6.

The Mark 7 had a greater maximum range over the Mark 6: 23.64 mi vs 22.829 mi. Interestingly, when firing the same conventional shell, the 16"/45 caliber Mark 6 gun used by the treaty battleships of the and es had a slight advantage over the 16"/50 caliber Mark 7 gun on the, when hitting deck armor—a shell from a 45 cal gun would be slower, meaning that it would have a steeper trajectory as it descended. At 35000 yd, a shell from a 45 cal would strike a ship at an angle of 45.2 degrees, as opposed to 36 degrees with the 50 cal.

Barrel life—the approximate number of rounds a gun could fire before needing to be relined or replaced—was 395 shells when using AP, increasing to 2,860 for practice rounds. By comparison, the 12"/50 caliber Mark 8 gun of the large cruisers had a barrel life of 344 shots, while the 16"/50 caliber Mark 7 gun fitted in the s had a barrel life of 290 rounds.

During World War II, the Mark 7 guns were only used for shore bombardment in the Pacific, while the Mark 6 guns also saw ship-to-ship combat in both the Pacific and European theaters. This was attributed to the fact that ships mounting the Mark 7 batteries, the, were commissioned later than the Mark 6-equipped and es, so they missed the Naval Battles of Casablanca and the Guadalcanal, one of the few instances where the US Navy's fast battleships were deployed for ship-to-ship combat. Most large scale naval battles involving the US Navy were fought by carrier-based aircraft in the Pacific.