12"/40 caliber Mark 3 and Mark 4
View of Ohio and her forward 12"/40 caliber guns in the Mark 4 turret in 1916.
Type	Naval gun
Place of origin	United States
Service history
In service	1902
Used by	United States Navy
Wars	World War I
Production history
Designer	Bureau of Ordnance
Designed	1899
Manufacturer	U.S. Naval Gun Factory
No. built	Mark 3: 41 (Nos. 15–41, 50–56) Mark 4: 10 (Nos. 49, 58–60, 150–154, 179)
Variants	Mark 3 and Mark 4
Specifications
Mass	116,480 lb (52,830 kg) (with breech) 114,960 lb (52,140 kg) (without breech)
Barrel length	40 ft 0 in (12.19 m) bore (40 calibers)
Shell	870 lb (390 kg) armor-piercing
Caliber	12 in (305 mm)
Elevation	Mark 4: −3° to +15° Mark 5: −7° to +20°
Traverse	−150° to +150°
Rate of fire	0.66 rounds per minute (as commissioned) 2 rounds per minute (after 1906)
Muzzle velocity	2,800 ft/s (850 m/s) (as commissioned) 2,600 ft/s (790 m/s) (first derating) 2,400 ft/s (730 m/s) (final derating)
Effective firing range	19,000 yd (17,374 m) at 15.5° elevation

The 12"/40 caliber gun (spoken as "twelve-inch-forty--caliber") were used for the primary batteries of the United States Navy's last class of monitors and the Maine-class and Virginia-class pre-dreadnought battleships.^[1]

The 12-inch (305 mm)/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, gun Nos. 15–48 and 50–56, 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, gun Nos. 49, 58–60, 150–154, and 179, was similar to the Mark 3 but with a smaller chamber for the reduced propellant charge.^[1][2]

The guns mounted in the Virginia-class battleships were in an unusual two-level turret with the 8-inch (203 mm)/45 caliber guns on top of the larger 12-inch guns. This arrangement ultimately proved unsuccessful but helped the Navy in the successful development of superfiring turrets later used in the dreadnought South Carolina.^[1]

Gun No. 49, while testing powder at the Naval Proving Ground, had the entire muzzle and chase blow off. The board appointed to investigate came to the conclusion that the new powder, while performing properly, caused a pressure along the chase that was dangerously close to the strength curve. It was decided that when the guns were withdrawn to be relined they would add an additional hoop that extended to the muzzle would be placed on the chase.^[3]

• Bluejackets Manual, 1917, 4th revision: US Navy 14-inch Mark 1 gun