12 " / 45 caliber Mark 5 gun The 12 " / 45 caliber Mark 5 gun was a US 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 12 " / 45 caliber Mark 5 naval gun was designed as an incremental improvement upon the preceding American naval gun , the 12 " / 40 caliber 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 53 tons and was capable of firing 2 to 3 times a minute . At maximum elevation of 15° it could fire an 870 - pound shell approximately 20 , 000 yards . 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 10 , 000 yards . With an initial muzzle velocity of 2 , 700 f / s , 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 16 . 6 inches of Harvey plated side armor at 6 , 000 yards , 12 . 2 inches at 9 , 000 yards , and 9 . 9 inches at 12 , 000 yards . By comparison the 12 " / 40 caliber Mark 4 it replaced could penetrate 14 . 6 inches , 11 . 6 inches , and 9 . 4 inches 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 - utilized 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 - " Mississippi " class battleships " Kilkis " and " Lemnos " , 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 with the Mark 5 were : In American service , the Mark 5 remained afloat ( albeit in dwindling numbers ) until 1930 , when the last guns were removed from the " Floridas " in compliance with the terms of the London Naval Treaty . Coastal Artillery . Following the signing of the Washington Naval Treaty in 1922 , many of the Mark 5 guns in service were removed from sea duty and transferred to the U . S . Army for use as coastal artillery . In this capacity , the maximum range of the Mark 5 increased to 30 , 000 yards , due to the greater elevation that was possible . These guns protected the Panama Canal Zone through World War II , after which some were permanently retired and some were sold to Brazil , where they might still be in use . In Greek service , the guns removed from the " Lemnos " were emplaced on the island of Aegina , where they helped to defend the approaches to the port of Athens .