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 barbette s 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 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 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 , 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 Mark 5 armor - piercing shell . However , the shell - handling system for these guns was redesigned to use the " super - heavy " 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 gun s 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 battleship s . 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 .