The 18"/48 caliber Mark 1 – United States Naval Gun was the initial name and design for a large caliber naval gun in the early 1920s. After the Washington Naval Treaty prohibited the development of guns larger than 16 in (406 mm), the gun was relined and finished as a high velocity 16"/56 Mark 4 gun. After the start of World War II, the gun was again relined to 18" and tested with a new Super Heavy Shell. The gun in its final form is currently displayed at the Dahlgren Naval Weapons Facility in Virginia.