The 16"/50 caliber Mark 2 gun and the near-identical Mark 3 were guns originally designed and built for the United States Navy as the main armament for the South Dakota-class battleships and s. The successors to the 16"/45 caliber gun Mark I gun, they were at the time among the heaviest guns built for use as naval artillery. As part of the Washington Naval Treaty of 1922, both of these ship classes were cancelled part way through construction, rendering surplus about 70 examples of the 16-inch/50 which had already been built. Twenty were released to the US Army, between 1922–1924, for use by the Coast Artillery Corps, the rest were kept in storage for future naval use. Only ten of the twenty available guns were deployed (in five two-gun batteries) prior to 1940. When the design of the began in 1938, it was initially assumed these ships would use the surplus guns. However, due to a miscommunication between the two Navy departments involved in the design, the ships required a lighter gun than the Mark 2/Mark 3, resulting, ultimately, in the design of the 16"/50 caliber Mark 7 gun. In January 1941 all but three of the remaining fifty Mark 2 and Mark 3 guns were released to the Army. They were the primary armament of 21 two-gun batteries built in the United States and its territories during World War II. However, none of these were fired in battle. Development The first example of a US 16-inch gun was an Army weapon, the M1895, approved for construction in 1895 and completed in 1902; only one was built. The first US Navy 16-inch gun was the 16-inch/45 caliber Mark 1 gun, which armed the Colorado-class battleships launched 1920–21. The second Navy design, the Mark 2, was intended as armament for the planned South Dakota-class battleships, and also selected for the modified design of the s, replacing the 14-inch/50 caliber gun that was originally used for the design. The Mark 3 was a slightly modified version of the Mark 2. With the United States entering into the Washington Naval Treaty, the terms limited the United States to a maximum displacement of . As both the South Dakota-class battleships and Lexington-class battlecruisers exceeded this limit, the Navy was required to cancel their construction, doing so in 1922. While two of the Lexington class were re-ordered as s, none of them were completed with