Until the Iowa-class battleships joined the fleet during World War II with their powerful Mark 7 guns, the 16″/45 (40.6 cm) Mark 6 was the biggest and best that the U.S. Navy had to offer. An improvement of the 1920s era 16-inchers used on the Colorado-class battleships, they were used (9 per ship in 3×3 mounts) in the six ships of the North Carolina and South Dakota-class dreadnoughts to heavy service in the war, being credited with knocking out at least two Axis battleships in surface combat and countless instances of naval gunfire support.
Bow turrets of USS North Carolina BB-55. Note CXAM-1 radar above the main battery director, which dates this photograph as being sometime shortly after 27 August 1941 U.S. Naval Historical Center Photograph # 80-G-K-13971 via Navweaps
A good number of these tubes still exist as North Carolina (BB-55), Massachusetts (BB-59), and Alabama (BB-60) all still exist as museum ships, mounting no less than 27 of these guns between them.
And you can add at least one more tube, recently discovered still at work for Uncle Sam.
From a February report by the Naval Heritage Command:
Bob Fish, author and USS Hornet Museum trustee, recently visited NASA’s AMES Research Center in Sunnyvale, CA, to investigate the possibility of cooperation and collaboration of STEM-related programming. While there, Bob visited the Hypervelocity Flight Test Facility with their engineers.He was then guided into the original 1960’s era hyper-velocity test lab which consisted of an old projectile acceleration tube that is now rarely used.
To his surprise, Bob noticed the inscription on the breach of the barrel read “US Navy.” It was in fact a Mark 6 16-inch battleship gun!
