18"/48 caliber Mark 1 gun

The gun in its final form is currently displayed at the Dahlgren Naval Weapons Facility in Virginia.

• Name — 18"/48 caliber Mark 1 gun • Origin — United States • Type — Naval gun • Is Ranged — yes • Is Artillery — yes • Used By — United States Navy • Design Date — 1920 • Number — 1

• Name — 16"/56 caliber Mark 4 gun • Origin — United States • Type — Naval gun • Is Ranged — yes • Is Artillery — yes • Used By — US • Production Date — 1927 • Number — 1 • Range — yard at 40° with 2,100 lb AP shell The 18"/48 cal Mark 1 was finished with an extra thick 16-inch liner, and the resulting weapon became the 16"/56 cal Mark 4. The gun was first tested at the Dahlgren proving grounds in 1927, with tests continuing into the 1930s. It fired a AP shell at muzzle velocity, with a range of yard at 40° elevation. The high muzzle velocity of the prototype resulted in very short liner life of only 45 rounds, with a predicted liner life of 125 full charge rounds for the service weapon. During the early stages of the design process for the, the gun was briefly considered for the main battery, but was rejected due to short liner life; the belt armor of the Montana class was also designed against the 16"/56 cal gun.

• Name — 18"/47 caliber Mark A gun • Origin — United States • Type — Naval gun • Is Ranged — yes • Is Artillery — yes • Used By — US • Production Date — 1942 • Number — 1 • Range — yard at 40° with 3,850 lb AP shell In 1938, with Japan's refusal to sign the Second London Naval Treaty and worsening situation in Europe and Asia, battleship studies once again considered 18-inch caliber main guns. During the early design stages of a 45,000-ton "slow" battleship alternative to the "fast" battleship design that would eventually result in the, the 18"/48 cal gun option was considered in several proposals. The 16"/56 cal Mark 4 had its threaded tip cut off and was then relined to 18-inches; this weapon was then designated as the 18"/47 cal Mark A and was used to test a "Super Heavy" AP shell, which the gun fired at a nominal muzzle velocity, with a range of yard at 40° elevation. The highest energy shot was with a 3,848-lb projectile fired at with a maximum pressure of 19.91 tons per square inch. Ultimately, the General Board decided that a 16"/50 cal gun offered the best combination of performance and weight, and the new 16"/50 caliber Mark 7 gun was used by the design. The preeminence of naval aviation ended further developments into large caliber naval guns. While the 18"/47 cal Mark A was used to test the new "Super Heavy" 18-inch AP shell, if the caliber had been selected, a new lightweight 18"/48 cal Mark 2 gun would have been built. The new lightweight 18-inch gun would have fired the 3,850 lb "Super Heavy" shell at muzzle velocity, with a design similar to the contemporary 16"/50 cal Mark 7 gun rather than the 1920s technology used in the 18"/47 cal Mark A construction. While the development of large caliber naval guns ceased after World War II, the gun was subsequently used for the development of low-drag aerial bombs. In this role, the gun fired a total of 114 shots, with the final shot fired on 7 November 1957. With the conclusion of testing, the gun was placed on display at the Dahlgren Naval Weapons Facility in Virginia.

The built-up gun is constructed of liner, a tube, jacket, nine hoops, six locking rings, a separate yoke ring and screw box liner. The breech mechanism was a down-swinging Welin block with vertical lever operating gear. Chromium plating of the bore was considered in the 1940s but never carried out.