The 12″/45 caliber Mark 5 gun was an American naval gun that first entered service in 1906. Initially designed for use with the of pre-dreadnought battleships, the Mark 5 continued in service aboard the first generation of American dreadnoughts. Design and development The /45 caliber Mark 5 naval gun was designed as an incremental improvement upon the preceding American naval gun, the 12"/40 caliber gun Mark 4. As such, it was a very similar weapon, having been lengthened by 5 calibers to allow for improved muzzle velocity, range, and penetrating power. Designed to the specifications of the Bureau of Ordnance, the Mark 5 was constructed at the U.S. Naval Gun Factory in Washington, D.C. Measurements and Capabilities The Mark 5 weighed and was capable of firing 2 to 3 times a minute. At maximum elevation of 15° it could fire an shell approximately . However, this range was largely academic at the time the gun was initially designed, as no rangefinding techniques had yet been developed capable of accurately firing beyond about . With an initial muzzle velocity of , the gun had a barrel life of 175 rounds, and was capable of firing either Armor Piercing or Common projectiles. As designed, the Mark 5 was capable of penetrating of Harvey plated side armor at , at , and at . By comparison the 12-inch/40 caliber Mark 4 it replaced could only penetrate , , and at those distances, respectively. Naval Service The Mark 5 entered service in 1906 and remained the primary battleship gun for all American battleships commissioned before 1912, at which point it was replaced by the 12"/50 caliber Mark 7. All told, the Mark 5 would arm 14 battleships of five different classes, making it the most-used main gun in American battleship history. Despite this distinction, the only Mark 5 guns ever to be fired in anger were actually in Greek, and not American, service. The ex-s and , sold to the Royal Hellenic Navy in 1914, fought in both the Allied Intervention in the Russian Civil War, and the Greco-Turkish War. Though during World War I the Mark 5 would cross the Atlantic for duty aboard two of the American battleships serving in the 6th Battle Squadron of the Grand Fleet, it was never fired in any engagement, as no battles were fought with the German High Seas Fleet in 1918. The five classes armed