History of the U.S. Navy’s Only 18-inch Gun
By James C. Poynor
The design for a Navy 18-inch gun began at the U.S. Navy Bureau of Ordnance sometime prior to 8 January 1917. A letter bearing that date states “The Bureau has under way designs for an 18"/45 caliber naval gun. The weight of shell now proposed is 3,000 pounds, muzzle velocity 2,600 foot seconds.”
Another letter, dated 19 April 1917 states that a blueprint of a proposed 18"/48 caliber gun has been completed (Sketch 8951) and that the projectile weight then being considered was 2,900 lbs. with an expected muzzle velocity of 2,700'/second. Two gun designs were considered: one was a light version and the other was a heavier version to allow removing the gun from a turret without dismantling it. The heavier version was chosen.
By early February 1919, a number of charge weights had been considered and it was determined that for all practical purposes, powder developed for the 16"/50 gun would suffice. By December 1919, the rough forging for the liner and possibly other rough forgings had been completed and material properties had been assessed. A letter dated 10 January 1920, stated that the Naval Proving Ground was of the opinion the heaviest weight projectile suitable for this gun was 3,300 lbs. at a velocity of 2,500'/second. Dahlgren currently has two 18-Inch Type B Target projectiles from circa 1921, and these weigh 3,330 lbs. each. There are also two 18-inch Type C Target projectiles from the same period at NSWC Crane, and these weight 2,900 lbs. each.
When the Washington Naval Treaty was signed in 1922, the 18-inch gun was approximately 50% completed. A decision had to be made as to whether it should be finished as designed or converted to a special 16-inch test gun of 56-caliber length. An earlier 16"/56 design existed so there was already an interest in testing a 16-inch gun of this length. A letter dated 8 April 1922 stated the Bureau of Ordnance had approved the cost estimate to convert this gun to a 16"/56 caliber gun ($20,000). The resulting new gun was designated the 16-inch 56-Caliber Type Gun No. 201. Later documents refer to this gun as the 16-inch Gun Mark IV No. 201.
On 26 April 1927 just before noon, the 16-inch gun number 201 was unloaded at Dahlgren. Proof firing commenced on 8 July 1927 and after seven rounds had been fired, the proof firing series was complete. The highest pressure shot out of those initial seven rounds resulted from using 810 lbs. of SPD-2254 powder and a 16-Inch Mark IV 2,101-lb. target projectile (like many, made by The Tredegar Co. of Richmond, VA). This combination yielded a peak pressure of 20.07 tons per square inch and a muzzle velocity of 3,105'/second. In this first configuration, the recoiling weight was 488,450 lbs. and the complete gun assembly weighed 586,300 lbs.
The standard service charge was determined to be 775 lbs. of powder in eight sections (bags) and this amount of propellant with a 2,100-lb. projectile produced a muzzle velocity of 2,994'/second. A 24 December 1927 letter lists the value of the complete gun and girder at $440,063.02. Another series of ranging shots were conducted on 1–2 August 1928 so that more wear and ranging data could be taken. Four shots fired at an elevation of 40° produced a range of 49,383 ± 184 yards. These shots were the longest (that we have records for) ever fired at Dahlgren. The maximum range of the 16-inch Mark IV gun would have been about 52,000 yards, and no other gun ever fired at Dahlgren could have out-ranged the 16"/56. On 24 June 1940, the final round was fired from this gun, bringing the grand total to 102 rounds fired.
In 1941, the 16-inch Mark IV gun was sent back to the Naval Gun Factory by barge for conversion to an 18-inch gun. The intention was to convert the gun back to the original 18"/48 configuration, but this could not be easily accomplished because threads had been cut at the muzzle during the 16-inch conversion. These threads had to be removed and, for this reason, the gun is now 47-calibers long and not the 48-calibers desired. The designation for this newly reconfigured gun was the 18"/47 Caliber Gun Mark A, No. 1. The weight of the complete gun assembly, as presently configured, is 559,974 lbs. The 18"/47 Mark A arrived at Dahlgren on 23 September 1941, and the first proof shot was fired on 4 February 1942 using a Type E-1 Tredegar Target Projectile. After correcting some problems with the counter-recoil system, proof firing was completed on 20 March 1942. The highest energy shot ever fired from this gun (376M foot-pounds) was with a 3,848-lb. projectile using 910 pounds of IHIC Sample 2 powder. This proof shot was the highest energy shot ever fired from an American gun. The muzzle velocity for this shot was 2,508'/second and the maximum pressure was 19.91 tons per square inch.
After the 21-round proof and ranging series was completed at the Main Battery, the gun was moved to the Plate Battery so that both armor penetration and armor-piercing projectile tests could be carried out. On 11 September 1942, the gun arrived at the Plate Battery and the first shot fired at this location was made on 3 June 1943. Between 3 June 1943 and 24 August 1945, five 18-inch Type B Mod 1 3,850-lb. armor-piercing projectiles were fired against armor plate targets. Five additional 18-inch type E-1 target projectiles were also fired during this period. A total of 31 18-inch projectiles were fired during the 18"/47 Mark A’s history.
18-inch gun being assembled at Plate Battery, 1954
Plate Battery. The 18-inch is the one with what appears to be a canvas cover over the breech and a plate shield on the front of the mount
With the rapid advance of military aircraft designs after the conclusion of WWII, the need for new bombs became evident. Aircraft speeds had increased to the point that externally carried bombs caused too much aerodynamic drag, and it became necessary to develop a series of new low-drag bombs. On 27 July 1951, the first 2,000-lb., low-drag bomb was test fired from the 18-inch gun against a 1.5" thick special treatment steel plate. Between 1951 and 1957, 21 2,000-lb., low-drag bombs and 62 750-lb. demolition bombs were successfully fired from this gun at various targets. The final shot was fired from the 18"/47 gun on 7 November 1957. Over its career as a test gun, the 18"/47 gun fired a total of 114 shots. When you see photographs of modern fighter jets carrying 2,000-lb., low-drag bombs, think of the Navy’s only 18-inch gun—the largest and most powerful American gun ever built!
The renderings depicted show the original design that was never completed. The current configuration differs only slightly though. Today, the barrel is 19-inches shorter, the yoke has been modified, and a counterweight has been attached to the front of the slide. During its conversion to an 18-inch gun in 1941, lightening cuts were made on both sides of the yoke and a slide counterweight was added so the gun would balance properly with a lighter and shorter barrel. All the components shown in these two renderings exist with the exception of a few minor parts. The missing parts can be made new and it is the author’s opinion this largest of American guns is worthy of being restored, reassembled and placed on public display at a suitable location. The history above was compiled from records of the Naval Surface Warfare Center, Dahlgren Division, VA, and the National Archives and Records Administration, Washington, D.C.
Side rear view
AP projectile, caption: 3,850-pound B-1 armor-piercing projectile