 A prototype Mark 2 was tested and proven on 8 April 1918 , and the gun was scheduled to go into naval service in 1923 . However , with the cancellation of both the Lexington and the South Dakota classes , no guns were installed on any ships even though 71 had been built and 44 were under construction . In 1922 – 24 , twenty of the guns were given to the Army for use as coastal defense guns along with the Army 's 16 " / 50 caliber M1919 guns . Later planning called for the use of these guns in the Iowa @-@ class battleships , but miscommunication between design bureaus led to the 16 " / 50 caliber Mark 7 gun being used instead . As a result , all but three of the Navy 's remaining Mark 2 and 3 guns were sent to the Army to also be used as coastal defense guns . 
