12"/40 caliber gun

The 12"/40 caliber gun (spoken "twelve-inch-forty--caliber") were used for the primary batteries of the United States Navy's last class of monitors and the and Virginia-class battleship pre-dreadnought battleships.

Design
The 12-inch/40 caliber gun was developed after the Spanish-American War to use the new smokeless powder that had recently been adopted by the Navy. The Mark 3 was constructed of tube, jacket, and eight hoops. It was found that the early guns suffered from excessive bore erosion, in an attempt to fix this the Navy reduced the propellant charges to reduce the muzzle velocity, because of this the Mark 4 was similar to the Mark 3 but with a smaller chamber for the reduced propellant charge.

Service history
The guns mounted in the Virginia-class battleships were in an unusual two-level turret with the 8-inch guns on top of the larger 12-inch guns. This arrangement ultimately proved unsuccessful but helped the Navy in in the successful development of superfiring turrets later used in the dreadnought USS South Carolina (BB-26).