The last two ships of the 1942 Naval Construction program were to have been battleships of about 90,000 metric tons armed with either eight or nine 51 cm (20.1") cannons. After some study - and perhaps a dose of reality - these ships were redesigned as slightly larger Yamato class ships armed with six main guns in three twin turrets. These ships were never officially ordered and all plans were destroyed at the end of World War II.

There is little information available for these guns, but it appears that two guns and one gun mount were ordered from Kure Navy Yard in June 1941. Construction was halted with the start of the Pacific War at which time the first gun was having its breech fitted and the second gun completed up to the 4A tube. The gun mount and the fittings were mostly complete but unassembled. All of these were left as they were and no further work was undertaken during the war. Construction was similar to that for the 46 cm (18.1") guns used on the Yamato class, being 1A, 2A, 3A, 4B, 5A and 5B tubes with wire winding. The breech was probably the Japanese version of the Asbury type with a Welin breech block, similar to the ones used on the Yamato class.

It is not known what happened to the guns and mounting, they were probably destroyed at the end of the war.

No formal designation seems to have been assigned to these guns. During development, they were informally known as A Gun (Kô Hô) Type. The bore diameter may have been purposely mis-stated, similar to how the 46 cm guns were officially called 40 cm guns. Type 98 (1938) is my estimate, but they may have been Type 94 if their breech mechanism design followed that of the 46 cm guns.

Gun Characteristics

Designation 51 cm/45 (20.1") "A" (Kô Hô) Type 98 (?) (Model 1938)
Official Designation: 45 caliber Type 98 51 cm Gun
Ship Class Used On A-150 Class (Warships # 798 and 799)
Date Of Design about 1938
Date In Service N/A
Gun Weight 223.4 tons (227 mt) with breech mechanism
Gun Length oa 927.6 in (23.560 m)
Bore Length about 900 in (22.840 m)
Rifling Length N/A
Grooves N/A
Lands N/A
Twist N/A
Chamber Volume N/A
Rate Of Fire about 1 - 1.25 rounds per minute


Type Bag
Projectile Types and Weights APC - 4,190 to 4,409 lbs. (1,900 to 2,000 kg)
HE - 4,097 lbs. (1,858 kg)
Bursting Charge APC - 67 to 70 lbs. (30 to 32 kg)
HE - 186 lbs. (84 kg)
Projectile Length N/A
Propellant Charge 1,058 lbs. (480 kg)
Muzzle Velocity N/A
Working Pressure N/A
Approximate Barrel Life N/A
Ammunition stowage per gun N/A

The propellant charge was in eight bags.

Mount/Turret Data

Designation Twin Turret
  Design A-150 (3)
Weight 2,736 tons (2,780 mt) 1
Elevation -5 / +45
Elevation Rate about 10 degrees per second
Train about +120 / -120 degrees
Train Rate about 2 degrees per second
Gun recoil N/A
Loading Angle N/A
  • ^By way of comparison, the Turrets on the Yamato weighed 2,730.2 tons (2,774 mt).
  • It was planned that these twin turrets would be about the same size as those on the Yamato class. The designers seem to have succeeded in this, as the roller path for the twin turret was 12.0 m compared to 12.274 m for the 46 cm three-gun turret. However, the recoil force was greater, which means that the structures of these ships would have needed to have been stronger in order to handle the additional stress.


"Naval Weapons of World War Two" by John Campbell
"Battleships: Axis and Neutral Battleships in World War II" by W.H. Garzke, Jr. and R.O. Dulin, Jr.
"Japanese Cruisers of the Pacific War" by Eric Lacroix and Linton Wells II
"The Yamato Class and Subsequent Planning" by Hans Lengerer and Lars Ahlberg
Special help from Nathan Okun

Page History

09 October 2006 - Benchmark
27 May 2012 - Updated to latest template
12 December 2015 - Added a few details
26 September 2016 - Converted to HTML 5 format
02 June 2019 - Added details on gun construction and turret notes