Description

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. The prototype for this weapon was about halfway completed when the Washington Naval Limitation Treaty of 1922 outlawed 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 showed that the same 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, even if the armor protection 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 prototype was finally retired just after World War II. It is now on display at the Dahlgren Naval Weapons Facility in Virginia, USA, as shown in the photograph 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.

Gun Characteristics

Designation 18"/48 (45.7 cm) Mark 1
16"/56 (40.6 cm) Mark 4
18"/47 (45.7 cm) Mark "A"
Ship Class Used On
(see Note)
None (Experimental)
Date Of Design 1920
Date In Service Never mounted aboard ship

Prototype:
  As 18"/48 (45.7 cm): Never used in this version
  As 16"/56 (40.6 cm): 1927
  As 18"/47 (45.7 cm): 1942

Gun Weight 18"/48 (45.7 cm): 177.8 tons (180.7 mt)
16"/56 (40.6 cm): 185.2 tons (188.2 mt)
18"/47 (45.7 cm): 177.0 tons (179.8 mt)
Gun Length oa 18"/48 (45.7 cm): 884.0 in (22.454 m)
16"/56 (40.6 cm): 915.0 in (23.241 m)
18"/47 (45.7 cm): 865.0 in (21.971 m)
Bore Length 18"/48 (45.7 cm): 864 in (21.946 m)
16"/56 (40.6 cm): 896 in (22.758 m)
18"/47 (45.7 cm): about 846 in (21.488 m)
Rifling Length 18"/48 (45.7 cm): 737.263 in (18.727 m)
16"/56 (40.6 cm): 765.712 in (19.449 m)
18"/47 (45.7 cm): 718.965 in (18.262 m)
Grooves 16"/56 (40.6 cm): 96 grooves
Others: N/A
Lands N/A
Twist 18"/48 (45.7 cm): Uniform RH 1 in 32
16"/56 (40.6 cm): Uniform RH 1 in 35
18"/47 (45.7 cm): Uniform RH 1 in 25
Chamber Volume 18"/48 (45.7 cm): 36,900 in3 (604.7 dm3)
16"/56 (40.6 cm): 34,000 in3 (557.2 dm3)
18"/47 (45.7 cm): 36,900 in3 (604.7 dm3)
Rate Of Fire about 1.5 - 1.75 rounds per minute

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.

Ammunition

Type Bag
Projectile Types and Weights All for AP Rounds:

18"/48 (45.7 cm) - 2,900 lbs. (1,315 kg)
16"/56 (40.6 cm) Mark 3 - 2,100 lbs. (954.5 kg)
18"/47 (45.7 cm) - 3,850 lbs. (1,746.3 kg)

Bursting Charge 16"/56 (40.6 cm) Mark 3 AP - 70 lbs. (31.6 kg)
18"/47 (45.7 cm) AP - about 68 lbs. (30.8 kg)
Projectile Length For 18"/48 (1921 design) - 72 in (183 cm)
For 16"/56 (40.6 cm) - about 64 in (163 cm)
For 18"/47 - about 81 in (206 cm)
Propellant Charge 18"/48 (45.7 cm) - 890 lbs. (403.7 kg) NC
16"/56 (40.6 cm) - 810 lbs. (367.4 kg) NC
18"/47 (45.7 cm) - 890 lbs. (403.7 kg) NC
Muzzle Velocity All for AP Rounds:

18"/48 (45.7 cm) - 2,700 fps (823 mps)
16"/56 (40.6 cm) - 3,000 fps (914 mps)
18"/47 (45.7 cm) - 2,400 fps (732 mps)

Working Pressure 18"/48 - 18.0 tons/in2 (2,835 kg/cm2)
16"/56 - 17.5 tons/in2 (2,760 kg/cm2)
18"/47 - 17.25 tons/in2 (2,720 kg/cm2)
Approximate Barrel Life
of 16"/56 (40.6 cm)
(see Note 2)
125 rounds at full charge 
225 rounds at reduced charge
Ammunition stowage per gun about 100 rounds
  1. 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.
  2. 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.

Range

Ranges of projectiles fired at new gun muzzle velocities
As 16"/56 (40.6 cm) With 2,100 lbs. (954.5 kg) AP Shell
Range @ 40 degrees 49,383 yards (45,156 m)
As 18"/47 (45.7 cm) With 3,850 lbs. (1,746.3 kg) AP Shell
Range @ 40 degrees 43,453 yards (39,734 m)

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.

Armor Penetration with 16" (40.6 cm) AP Shells

Range Side Armor Deck Armor
28,800 yards (26,335 m) 13.0" (330 mm) 3.5" (90 mm)

This data is from "U.S. Battleships: An Illustrated Design History" and is based upon the USN Empirical Armor Penetration formula. It was prepared by BuOrd in 1926 and is based upon a muzzle velocity of 3,000 fps (914 mps).

Armor Penetration with 18" (45.7 cm) "Super Heavy" AP Shells

Range Side Armor Deck Armor
25,000 yards (22,900 m) --- 6.25" (159 mm)
30,500 yards (27,900 m) 16.0" (406 mm) ---

This data is from "U.S. Battleships: An Illustrated Design History" and is based upon the USN Empirical Armor Penetration formula. BuOrd estimated that a ship with 16 inch (40.6 cm) side armor and 6.25 inch (15.9 cm) deck armor would have no immune zone whatsoever from the "super-heavy" 18" (45.7 cm) AP projectile.

Mount/Turret Data

It was estimated that a three-gun turret fitted with the 18"/48 (45.7 cm) would have weighed about 3,000 tons (3,050 mt), which is over 40% heavier than the three-gun 16"/50 (40.6 cm) turrets that are carried by USS Iowa (BB-61). In addition, the barbettes and supporting framework would have been proportionally heavier as well. From these figures, it can be seen that a battleship the size of USS Iowa (45,000 standard tons) would have been able to carry no more than five 18"/48 (45.7 cm) guns vs. the nine 16"/50 (40.6 cm) guns that she actually did carry.

Additional Pictures

Sources

"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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Special help from Nathan Okun and James P.

Page History

25 May 2007 - Benchmark
05 January 2016 - Added blueprint
23 June 2016 - Converted to HTML 5 format