These three designations were all assigned to a single gun used for experimental testing purposes between 1927 and 1945. In the early 1920s, the USN was in the process of developing the 18"/48 (45.7 cm) Mark 1 naval gun. 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 for development.

This prototype weapon was about halfway completed when the Washington Naval Limitation Treaty of 1922 banned guns larger than 16" (40.6 cm). As a result of the treaty, the USN decided to complete the prototype as an extra-long 16" (40.6 cm) weapon. This conversion was performed by grinding down the muzzle end, re-threading the barrel to hold an extension ring and inserting a thicker, 16" (40.6 cm) diameter liner. The prototype was then designated as the 16"/56 (40.6 cm) Mark 4 high-velocity test gun. Proofing began at the Dahlgren proving ground in July 1927 and the gun was fired throughout the 1930s in various tests.

Battleship design studies conducted in 1927-28 and again in 1938 considered using the 18"/48 (45.7 cm) gun, but the USN rejected it both times for the following reasons: 1) Excessive weight, 2) Very short liner life, 3) Lack of sufficient angle of descent for all but very long ranges - i.e., little deck penetration capability. The use of this weapon would also have violated all of the Naval Limitation Treaties of the 1920s and 1930s, possibly the most important factor in its rejection.

Tests with this weapon in the 1920s and 1930s convinced BuOrd that standard 18" (45.7 cm) AP projectiles had only marginally better armor penetration than 16" (40.6 cm) shells. In addition, design studies for what became the Iowa class (BB-61) battleships showed that this size battleship could carry a maximum of only six or seven of the much heavier 18" (45.7 cm) guns vs. nine 16"/50 (40.6 cm) guns, assuming that the armor protection and speed remained the same. The larger gun would also have had a slower rate of fire, as the heavier shells required would have been more difficult to handle. These conclusions led to the development of "Super-Heavy" AP projectiles in the late 1930s, which made the US guns using them superior to other nation's weapons of the same caliber.

The start of World War II removed all treaty limitations and revived interest in larger caliber weapons. In 1941, the threaded tip was cut off and a new 18" (45.7 cm) liner installed. This weapon was then redesignated as the 18"/47 (45.7 cm) Mark "A" test gun and "Super-Heavy" 18" (45.7 cm) AP projectiles were designed for it. The gun was first test fired in this new configuration in February 1942 and a few more times until the end of the war. The eclipse of the battleship by carrier aircraft greatly reduced the Navy's interest in pursuing further development of large caliber weapons and the last 18" (45.7 cm) projectile firing took place on 24 August 1945. The weapon was then used to test aircraft bombs before being fully retired in 1957. It is now on display at the Dahlgren Naval Weapons Facility in Virginia, USA, as shown in the adjacent photograph. For an expanded history of this weapon, please see "Other Resources" below.

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.

It is likely that if the 18"/48 (45.7 cm) gun had been chosen in 1938 to arm what became the USS Iowa (BB-61) class battleships that its design would have been changed to reflect the technology improvements of the 1920s and 1930s such that the resulting gun would have been of lighter weight than the original design. As mentioned above, the gun carriage design would have allow the guns to be removed and replaced without dismantling the gunhouse.

• ^A letter dated 8 April 1922 stated that the Bureau of Ordnance had approved the cost estimate of $20,000US to convert the prototype 18"/48 (45.7 cm) Mark 1 gun to a 16"/56 (40.6 cm) caliber gun. The resulting new gun was designated as the 16-inch 56-Caliber Type Gun No. 201. Later documents refer to this gun as the 16-inch Gun Mark IV No. 201.
• ^Although this weapon was extensively considered in numerous battleship design studies of the 1920s and 1930s, it was never formally selected for any ship. The last US battleship design, the cancelled USS Montana (BB-67) class, would have carried the same 16"/50 (40.6 cm) Mark 7 guns as did the previous USS Iowa (BB-61) class.

• ^Jim Poynor comments:

  "There were four projectile types designed in 1921:
      Type A: 3,330 lbs. (1,510 kg) (4.5 calibers)
      Type B: 3,330 lbs. (1,510 kg) (4.0 calibers)
      Type C: 2,900 lbs. (1,315 kg) (3.5 calibers)
      A blunt-nosed 3,330 lbs. (1,510 kg) proof slug

  Two of the Type B’s are still at NSWC Dahlgren and NSWC Crane has two of the Type C’s. To the best of my knowledge, no Type A’s exist and none may have ever been made. This also goes for the B-1’s and the E-1’s developed during the 1940s although there are E-1’s fired as test shots that are still in the Potomac River. All of the 1921 variants, except for the proof slug, were target projectiles."

• ^The 16"/56 (40.6 cm) Mark 4 version of the prototype weapon had an actual liner life of 45 rounds. The figures given above are the estimated life of the projected service version.
• The figures given above for the 18"/48 (45.7 cm) Mark 1 version are approximate, as the gun was never fired in this configuration.

The above ranges were the longest ever recorded with the prototype. However, this does not mean that these ranges would have actually been achieved in service.

• ^As 18"/48 (45.7 cm) guns were never selected for any ship, the values used in this table come from preliminary design studies of the 1930s and should be used with caution.
• ^Based upon other USN large caliber turrets of this time period, the final designs probably would have been significantly heavier.

"History of the U.S. Navy’s Only 18-inch Gun" by James C. Poynor

"Naval Weapons of World War Two" by John Campbell
"U.S. Battleships: An Illustrated Design History" and "US Naval Weapons" both by Norman Friedman
"Battleships: United States Battleships, 1935-1992" by W.H. Garzke, Jr. and R.O. Dulin, Jr.
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"Yes, there is an 18-inch Gun" letter to the editor by Amos A. Cleary, PAO, Naval Weapon Laboratory, Dahlgren, Va, as published in "All Hands" June 1968
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Special help from Nathan Okun and James Poynor

25 May 2007 - Benchmark
05 January 2016 - Added blueprint
23 June 2016 - Converted to HTML 5 format
23 November 2017 - Corrected caption for photograph of 18" AP
19 December 2018 - Added Other Resources and images from James Poynor
12 February 2019 - Added Jim Poynor comments in the Gun Characteristics and Ammunition sections
21 March 2021 - Changed link for Dahlgren article to point to Wayback Archives
02 December 2021 - Redid photograph of 18in gun at Dahlgren, VA, changed link to "All Hands" letter to point to Wayback Archives
08 January 2023 - Dated photograph of guns at Dahlgren, the scrapping of shown guns in that photograph and minor changes to description section