16-inch/45-caliber Mark 6 gun
Introduction The '''16"/45 caliber Mark 6 gun''' is a naval gun designed in 1936 by the United States Navy for their Treaty battleships. It was 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 three-gun 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  shell with a maximum  range and their turret limit of 30-degree elevation. A major alteration from the older guns was the Mark 6's ability to  fire a new  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 (700 m/s); at a reduced charge, the same shell would be fired at 1,800 f/s (550 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 elevation 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 barrel was  long overall, which is 45 bore diameters, hence the 16"/45 caliber; its bore length was  and rifling length was . Maximum range was obtained at an elevation of 45 degrees. With the heavy AP shell the maximum range was , and with the lighter  high capacity (HC) shell, . 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 , 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.alt=The turret held three guns; below the gun deck were the turret floor, the machinery floor, and two levels of magazines for the shells and propellant charges. Two centrally located ammunition hoists carried the ordnance from the magazines to the gun deck.
Shell The Mark 6 and 7 guns were originally intended to fire the relatively light  (1.00 long ton) 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 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  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, the battleship  employed these 16"/45 caliber guns as her primary armament, and she is believed to have fired the United States' first and last 16-inch shells of World War II; the first use occurring on 8 November 1942 during the Naval Battle of Casablanca (shortly before the Naval battle of Guadalcanal), the last on 9 August 1945 off the coast of Hamamatsu, Japan. Furthermore, their use at Casablanca was the only time that a fast battleship of the US Navy fired its guns in anger in the European theater, which was also one of the two engagements in World War II where the US Navy's fast battleship dueled an enemy battleship. ''Massachusetts'' heavy 16-inch AP shells caused significant damage to the incomplete battleship  although few of the shells actually exploded because they had been fitted with fuzes manufactured in 1918.One of ''Massachusetts'' hits penetrated both of ''Jean Bart's'' armor decks and exploded in the empty magazines for the missing 152 mm guns, had this magazine been full of propellant charges the resulting explosion could have destroyed the vessel, while ''Massachusetts'' fifth salvo jammed the rotating mechanism of ''Jean Bart's'' sole operational main battery turret.In the second instance, as the primary armament of  these guns were employed against the Imperial Japanese Navy's  (a much older and less powerful ship, armed with 8 × 14-inch guns and originally built as a battlecruiser during World War I) 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 ''Washington'' had the aid of a naval fire control computer—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 (although they did have complex mechanical ballistics computers, which had been in use since World War I). ''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  at night. Using her nine 16"/45 caliber Mark 6 guns, ''Washington'' fired 75 rounds of 16-inch AP shells and scored an incredible twenty heavy-caliber hits that critically damaged the ''Kirishima'', which eventually sank. During the same battle, ''South Dakota'' also fired off several salvos from her 16"/45 guns before she had to withdraw for repairs due to a faulty circuit breaker.
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  would only allow the addition of s over the ''South Dakota''s. Rather than retaining the 16"/45 caliber Mark 6 gun used in the ''South Dakota''s, 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 turret weighed some  more than the Mark 6 45 caliber did; the barbette size also had to be increased so the total weight gain was about , putting the ship at a total of —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, which weighed 192,310 pounds (87,230 kg).The Mark 7 had a greater maximum range over the Mark 6:  vs . 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 , 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. Most World War II large scale naval battles involving the US Navy were fought by carrier-based aircraft in the Pacific, and so the chief use of the US Navy's battleship guns was shore bombardment. Nonetheless, the Mark 6 guns saw ship-to-ship combat in 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 Guadalcanal, two of the few instances where the US Navy's battleships were deployed to fight other battleships. Operation Hailstone was the only instance when the Mark 7 guns were fired purely at surface ships, against two light cruisers and three destroyers, in a somewhat controversial surface action (since US Navy carrier aircraft could have achieved similar results).
See also * 40.6 cm SK C/34 gun – German equivalent* BL 16 inch Mk I naval gun – British equivalent* 41 cm/45 3rd Year Type naval gun – Japanese equivalent
References
Bibliography * * *
External links *  NavWeaps, ''16"/45 (40.6 cm) Mark 6'', navweaps.com, retrieved 21 Aug 16.