The 46 cm/45 (18.1") cannons used on the Yamato class were the most powerful guns ever installed on a battleship. While closely matched by the USA 16"/50 Mark 7 at long ranges, in a close-range engagement the penetration power of this weapon was unsurpassed. The muzzle blast is said to have been able to rip the clothes off personnel who were standing too close when the guns were fired, but this story is probably apocryphal. Blast pressure measured during trials at Kamegakubi test range was 7.0 kg/cm² (100 psi) at a point 15 m (50 feet) in front of the muzzle. This was twice as high as that for the 41 cm guns used on the Nagato class.

These guns were officially designated by the Japanese as 40 cm/45 Type 94 (15.7 inch) in an effort to hide their actual size, which was a closely-guarded secret until after the end of World War II.

During design discussions in 1934-35, the option of building 50 caliber guns was seriously considered. These would have given an MV 40 mps (131 fps) higher than the 45 caliber gun and a 2,600 m (2,850 yards) greater maximum range when using the same projectile and propellant loading. Time of flight to 20,000 m (21,870 yards) would have been 1.9 seconds shorter with the longer gun. In the end, it was decided that the additional weight of these longer guns was not offset by their greater power and the Japanese decided to stay with 45 caliber guns.

Yamato is known to have fired at enemy ships on only one occasion. This was at the Battle off Samar in October 1944 against the U.S. Taffy 1 and Taffy 3 escort carrier groups, with rounds possibly hitting USS Gambier Bay (CVE-73). Musashi is known to have fired her guns in anger only once when she used "sankaidan" (incendiary shrapnel) anti-aircraft shells during the Battle of the Sibuyan Sea in October 1944. One of these reportedly exploded in the barrel, disabling the gun.

Dispersion was very small for these guns with spreads of 440 to 550 yards (400 to 500 m) at maximum range being recorded. American reports from the Battle of Samar also comment on the small patterns.

A total of 27 guns were produced, with the first one being completed in March 1938 and tested at the Kamegakubi proving grounds. Eighteen of these guns were lost with Yamato and Musashi, two test guns at Kamegakubi were demolished in November 1945 in accordance with the general disarmament orders of the U.S. Army and the remaining seven were found in various stages of completion on the beach in a cove north of Kamegakubi. Five of these remaining guns were destroyed while the last two, No. 23 and No. 27, were taken to Dahlgren Proving Grounds in Virginia, USA, for testing, arriving on 6 and 7 June 1946. Two slides for these guns were later delivered to Dahlgren on 7 May 1948. The guns and slides were reportedly cut up for scrap sometime during the 1950s. Two partially completed turntables intended for Shinano were also captured and later destroyed.

These guns had an unusually complex construction, perhaps reflecting the difficulty in manufacturing such a large caliber. The A tube, designated as 2A, had the 3A tube shrunk on for somewhat over half the length from the breech end. This assembly was then wire-wound and had a layer of two tubes shrunk on for the entire length, followed by a two-part jacket at the breech end. The various tube locating shoulders were fitted with Belleville spring washers, presumably to lessen stress concentration and potential "steel choke" problems. This feature was similar to many Vickers designs which used cannelured rings to reduce this problem. The inner A tube, known as 1A, was radially expanded into place by applying hydraulic pressure in three separate operations. The inner A tube was rifled after it was in place. There were also a short breech ring and a breech bush screwed into the 3A tube. The breech mechanism was a Japanese version of the Asbury type with a Welin breech block.

A great disadvantage of this type of construction was that the gun could only be relined by completely boring out the inner A tube. This was so expensive a process that it was considered to be more practical to simply replace a worn out gun with a new one, although it does not appear that either battleship was ever regunned during the war. This may be seen as a reflection of the brief combat life of these ships.

• ^At the loading angle of +3 degrees, a firing cycle of about 30 seconds could be achieved. However, this would equate to a range of no more than 6,560 yards (6,000 m). The additional elevation and depression times required to reach an elevation of 41 degrees increased the firing cycle by about 11 seconds. As can be seen in the Range Table below, most ship-to-ship actions would rarely exceed an elevation of 20 degrees, so an intermediate time of 35 seconds would seem to be reasonable for most battle-range engagements. See the Firing Cycle Table image below for more information.

• ^Type 91 shells were all 6/∞ crh with boat tail. The diameter of the bourrelet was 18.07 in (45.898 cm). The original Type 91 APC was L/4.25 or 76.9 inches (195.35 cm) long. The later Type 1 version was L/4.47 or 81 inches (206 cm) long. The main difference was the length of the windscreen, as the conic angle went from 23.5 degrees to a longer, sharper 21 degrees. Type 1 shells also introduced a dye bag, which may have slightly increased their weight over that of the Type 91. The base of the Type 1 was slightly different from the Type 91 as the original two driving bands were replaced by a single double-width band with a copper ring in the middle of the band. The modified type superseded the original in 1941, so these battleships may never have carried any but the newer shell. See below for photographs of Target Shells made from APC Type 91 shells and a dimensional sketch of the APC Type 91.
• ^Common Type 0 HE was supplied with time fuzes for AA defense. US Naval Technical Mission to Japan report O-19 says that this round had an effective radius of 74.4 yards (81 m) but this seems to be optimistic.
• ^IS is my abbreviation for the Type 3 Common incendiary shrapnel round (sankaidan) intended for AA use. Fuzes for these were set in the shell handling room with fuze protectors used to prevent damage before loading. Type 3 Common may have made up as much as 40% of the outfit by 1944. See below for more information.
• ^110 DC₁ propellant was 11.0 mm in diameter and 370 mm long (0.433 x 14.6 inches).
• ^The propellant charge was in six bags. Bags were made of wool until 1942 at which time silk ones were substituted. Bags had a 1.1 lbs. (0.5 kg) separate silk bag of gunpowder at one end.
• ^Smaller propellant charges were available for training and target rounds. These were designated as "Reduced" and "Weak" charges. "Reduced" charges gave about two-thirds of the normal service velocity and were rated at 0.5 ESR while "Weak" charges were rated at 0.06 ESR. No muzzle velocity is given for "Weak" charges. Reduced charges were about 3/4 the weight of a full charge while Weak charges were about 1/2 the weight of a full charge. For proofing purposes, there was also a "Strong" charge that was about 20% larger than a full charge. This was rated at 2.0 ESR.
• ^Each mounting held 300 rounds with 180 rounds stored in two handling rooms on the rotating structure and the remaining 120 in shell rooms. The practice was to feed projectiles from the lower handling room until the supply was exhausted and then feed from the upper handling room.
• The Yamato class were issued the following dye colors for their Type 1 APC shells:
        Yamato: None (White)
        Musashi: Green

As were most Japanese warships, Yamato and Musashi were provided with a special anti-aircraft incendiary shrapnel shell officially designated as 3 Shiki tsûjôdan (Common Type 3) and supposedly nicknamed "The Beehive," but this could be apocryphal. This round weighed 2,998 lbs. (1,360 kg) and was filled with 900 incendiary-filled tubes. A time fuze was used to set the desired bursting distance, usually about 1,000 meters (1,100 yards) after leaving the muzzle. These projectiles were designed to expel the incendiary tubes in a 20 degree cone extending towards the oncoming aircraft with the projectile shell itself being destroyed by a bursting charge to increase the quantity of steel splinters. The incendiary tubes ignited about half a second later and burned for five seconds at 3,000 degrees C, producing a flame approximately 5 meters (16 feet) long.

The concept behind these shells was that the ship would put up a barrage pattern through which an attacking aircraft would have to fly. However, these shells were considered by US Navy pilots to be more of a visual spectacular than an effective AA weapon.

• ^These mountings had a unique sliding cross-head for the elevating cylinder anchor point. This could be set to allow either an elevation range of -5 to +45 degrees or a range +3 to +41 degree elevation. The advantage of this selection was that the latter setting meant that no locking pins had to be inserted to hold the guns at the loading angle, the operator merely kept the controls at the "depress" position until an indicator showed him that loading was complete. This sped up the loading cycle and, as shown in the range table above, cost very little in terms of maximum range.
• ^"US Naval Technical Mission to Japan report O-45 (N): Japanese 18" Gun Mounts" states that the planned rate of elevation was 6 degrees per second, but 8 degrees per second was obtained in practice.
• ^A Note on Sources: "Anatomy of the Ship: The Battleship Yamato" states in the text that the elevation rate was 10 degrees per second. However, Japanese sources agree with the 8 degree figure and this work makes extensive use of "US Naval Technical Mission to Japan report O-45 (N): Japanese 18" Gun Mounts" including using the Firing Cycle Table from that document that uses the 8 degree per second elevation rate. For these reasons, I believe that the 10 degree figure is an error.
• These turrets had two independent sets of training gear, only one of which was in use at a time. Each was driven by a 500 hp vertical swashplate motor driving a train of straight-toothed pinion gears. Guns were individually sleeved.
• The shell and handling rooms were above the magazines. These were located around both the upper and lower propellant handling rooms. The lower handling room supplied only the center guns while the upper handling room supplied the outer guns. 180 projectiles (60 per gun) were stored vertically on two levels of the rotating structure while the rest were in the fixed structure and fed only the lower shell handling rooms. A push/pull geared mechanism moved the shells around in these compartments.
• The gun axes were 137.8 in (350 cm) apart.
• Most references state that the center gun had a 0.080 second firing delay to reduce shell dispersion. This is from "US Naval Technical Mission to Japan report O-45 (N)." However, more recent works state that the center gun had a 0.300 second delay. This would result in a separation of about 755 feet (230 m) between the shells from the outer guns and the shell from the center gun.
• Each gun had its own pusher shell hoist and cordite cage hoist. Shells were delivered vertically to the gunhouse and then tipped over for ramming. The cylindrical flashtight charge container held all six charges in line, allowing a single rammer stroke to load the gun. Two auxiliary hoists were fitted in each turret. Both shells and propellant bags were hoisted vertically in suitable containers. Transfer of shell in the gunhouse was effected manually by overhead travelers and chain purchase.
• Armor thickness for the Yamato class as given in "Anatomy of the Ship: The Battleship Yamato" by Janusz Skulski:
      Face: 25.6 in (65 cm)
      Sides: 9.8 in (25 cm)
      Rear: 7.5 in (19 cm)
      Roof: 10.6 in (27 cm)
• The following excerpt is taken from US Naval Technical Mission to Japan report O-45 (N):

  The 46 cm (18-inch) triple mount was the first large mount ever designed and produced by the Japanese which was not practically identical with the large British mounts prior to World War I and built for the battleship KONGO. In almost every part of the 18-inch mount a complete departure from this old design was made. For shell and powder hoists the Japanese 18-inch mount [sic, but probably meant 16-inch mount] was copied, but, generally speaking, the mechanisms were unique. Compared with British and U.S. practice, (16-inch, 14-inch and 8-inch), the mount was simple in design. It had no complicated hydraulic safety interlocks and comparatively few mechanical ones. Reliance was placed on good drill to avoid accidents. Even taking into account the size of the gun, the general impression gathered is that an unduly large factor of safety had been allowed in the design of the turret machinery as a whole, resulting in a very heavy mount, the total revolving weight of one turret being 2,510 metric tons [2,470 tons].

  A very satisfactory rate of fire was obtained for a gun of this size: 1.5 rounds per minute at full elevation. A maximum range of slightly under 46,000 yds was obtained with a 3,220 lb shell.

  The most interesting features of the mount were (a) the method of stowing and moving shells about the shell and shell-handling rooms using a fairly simple, but bulky and heavy mechanism; (b) the powder cage and rammer, designed to enable a full charge to be loaded by a single rammer stroke; and (c) the attachment of the elevating piston rod to the slide which was designed to avoid the necessity for a complicated slide locking gear, and to reduce the loading cycle time by cutting out the time usually required for locking and unlocking the slide.

  Other mechanisms worthy of note are as follows:

      1) The powder bogie and mechanisms for transferring powder from the fixed to the moving structure.
      2) The gunhouse bogie and rammer.
      3) The wormless training gear, with its "coaster" gear substitute for the normal friction discs.
      4) The electric cable leading-in gear.

  It was stated by the Japanese that considerable attention had been paid to flashtightness throughout the turret. Although this was probably not up to U.S. and British standards, it is difficult to give a definite opinion on this point without having seen a completed turret. The performance of the turret in service was considered by the Japanese to have been very satisfactory with the exception of the large quantities of lubricating oil, which the Japanese could ill afford, were required by the training and powder hoist gears. Spreads of salvos were reasonably small, (about 500 to 600 yards at maximum range). The blast effect, particularly on the bridge, was found to be very severe.

  Without having seen one of these turrets in operation or even a completed turret, an opinion will not be expressed on its probable performance or its value as a weapon beyond that already given above. The statements of the Japanese, however, are considered to be well-founded.

"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.
"The Big Gun: Battleship Main Armament 1860-1945" by Peter Hodges
"The Yamato Class and Subsequent Planning" by Hans Lengerer and Lars Ahlberg
"The World Wonder'd: What Really Happened Off Samar" by Robert Lundgren
"Anatomy of the Ship: The Battleship Yamato" by Janusz Skulski
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US Naval Technical Mission to Japan report O-19: Japanese Projectiles General Types
US Naval Technical Mission to Japan report O-45 (N): Japanese 18" Gun Mounts
NPG Report No. 269: Ballistic Test and Metallurgical Examination of Japanese 8", 14", 16" and 18" Armor Piercing Projectiles
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Special help from Mike Connelley, Nathan Okun, Robert Lundgren and Richard Worth
Photographs and data on Gun No. 23 from LSM-362 and used here by the kind permission of Jack H. Carter, Jr.

23 April 2007 - Benchmark
27 May 2012 - Updated to latest template
08 January 2013 - Added note about incomplete guns and photograph of incomplete guns on beach
24 August 2014 - Added Turret Armor note
16 May 2015 - Added photograph and newspaper article showing Musashi firing
11 July 2015 - Added splash colors
27 August 2015 - Redid photograph of guns on beach and added link to source
01 November 2015 - Added photograph of projectiles
15 December 2015 - Minor changes
08 June 2016 - Converted to HTML 5 format, added photograph of Yamato at Truk
31 October 2016 - Added detail on ammunition handling
10 May 2017 - Added photographs and data on guns transported to Dahlgren, Virginia
04 June 2018 - Reorganized notes
15 October 2018 - Corrected elevation rate and added sketch of triple mount and firing cycle table
21 March 2019 - Added note regarding time fuzes on Common Type 0 HE
30 May 2019 - Added notes about blast pressure, 50 caliber gun, dispersion, propellant grain size, propellant charge sizes, ESR and center gun delay time
30 April 2020 - Minor changes for clarity and ease of use
29 September 2023 - Fixed broken link to point to Wayback Archive
09 December 2023 - Added sketch of Type 91 APC
19 April 2024 - Minor formatting changes