12-inch/40-caliber gun
              < 12-inch 
 
            
          | 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 | 
  | 
| Variants | Mark 3 and Mark 4 | 
| Specifications | |
| Mass | 
  | 
| 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 | 
  | 
| Traverse | −150° to +150° | 
| Rate of fire | 
  | 
| Muzzle velocity | 
  | 
| 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.