20 cm/50 3rd Year Type naval gun

Third year type 20 cm/50 caliber guns (五十口径三年式二〇糎砲) formed the main battery of Japan's World War II heavy cruisers. These guns were also mounted on two early aircraft carriers. The typical installation was ten 20 cm/50 guns; although Tone class cruisers carried eight while Furutaka and Aoba class cruisers and, after modernization, Akagi carried only six.

These were built-up guns with an inner A tube, encased by a second tube, encased by a full length jacket. Early guns were partially wire-wound, but later guns dispensed with the wire winding. The guns were breech loaded with two cloth bags of smokeless powder. Third year type refers to the Welin breech block on this gun. Breech block design began in 1914 AD, the third year of the Taishō period. This breech block design was also used on Japanese 40 cm (16 inch), 15.5 cm (6 inch), 14 cm (5.5 inch), 12.7 cm (5 inch), and 12 cm (4.7 inch) naval guns.

1 GÔ (Mark I) guns
The first model of this gun used a 32.63 kg powder charge to fire 20 cm projectiles weighing 110 kg at a velocity of 870 m/s. Useful life was 300 effective full charges (EFC) per gun. These guns were initially installed in type A low-angle (25°) single mounts aboard Furutaka-class cruisers, in type C (40°) twin turrets in the Aoba class, and in type D (40°) twin turrets in the Myōkō class. Mark I guns can be visually distinguished from Mark II guns by an abrupt step in the chase diameter which was absent on the latter guns.

2 GÔ (Mark II) guns
Second model guns used a 33.8 kg powder charge to fire 8 in projectiles weighing 125.85 kg at a velocity of 835 m/s. Rate of fire varied from four or five rounds per minute firing at low angles diminishing to two or three rounds per minute firing at maximum elevation. These guns had a useful life expectancy of 320 to 400 EFC. These guns and the type E twin turret with 70 degree elevation installed on Takao class cruisers were influenced by Royal Navy County class cruisers. Type E turrets were promptly redesigned to limit elevation to 55 degrees when 70 degree elevation proved impractical. Modified type E turrets were installed as original equipment aboard Maya and the Tone class cruisers, and replaced the original turrets aboard Furutaka and Mogami class cruisers. Mark II guns replaced the original Mark I guns in type C and D turrets so all Japanese heavy cruisers carried Mark II guns in twin turrets by December 1941.

Aircraft carrier installations
Mark I guns were installed in casemates with a maximum elevation of 25 degrees limiting maximum range to 22 km. Aircraft carriers originally had four guns originally mounted in two type B twin turrets with a maximum elevation of 70 degrees. These were transferred to casemates in Kaga in 1934 and simply removed from Akagi in 1936.

Ammunition
Shell weights and muzzle velocities for high explosive or incendiary shrapnel are specified for each of the Marks above. Infobox muzzle velocity applies to type 91 armor-piercing (AP) shells with trajectory information below. Illuminating shells were fired with a reduced charge at a muzzle velocity of 710 meters per second (2330 ft/sec).

Type A
Only Mark I guns were installed in type A mounts. Maximum elevation was 25° in the six single mounts installed aboard Furutaka class cruisers in 1926, in the six casemate mountings installed on aircraft carriers Akagi in 1927 and Kaga in 1930, and in the four casemate mountings added to Kaga in 1934.

Type B
Only Mark I guns were installed in type B mounts. Maximum elevation was 70° in the two twin turrets installed aboard Akagi in 1927 and Kaga in 1930. These turrets were removed from Kaga in 1934 and from Akagi in 1936. Guns removed in 1934 were transferred to additional casemates aboard Kaga, but those removed in 1936 were not replaced.

Type C
Three twin turrets with maximum elevation of 40° were installed only aboard Aoba class cruisers. The Mark I guns installed in 1927 were replaced by Mark II guns in 1937 and 1938.

Type D
Five twin turrets with maximum elevation of 40° were installed only aboard Myōkō class cruisers. The Mark I guns installed in 1928 and 1929 were replaced by Mark II guns between 1931 and 1934.

Type E
Only Mark II guns were installed in type E twin turrets. Maximum elevation was 70° in the five turrets installed aboard Atago, Takao, and Chōkai in 1932. Early recognition of the impracticality of using these guns for anti-aircraft fire caused reduction of maximum elevation to 55° in all subsequent installations. Maya received five 55° turrets as original equipment in 1932. Three new turrets with Mark I guns from Myōkō class cruisers re-bored to Mark II replaced the original type A mounts aboard Furutaka class cruisers in 1936 and 1937. Tone class cruisers were completed with four type E turrets in 1937 and 1938. Five new turrets replaced the original triple 6-inch turrets aboard Mogami class cruisers between 1939 and 1941.

Weapons of comparable role, performance and era

 * 203mm/50 Modèle 1924 gun French equivalent
 * 20.3 cm SK C/34 Naval gun German equivalent
 * 203 mm /53 Italian naval gun Italian equivalent
 * BL 8 inch Mk VIII naval gun UK equivalent
 * 8"/55 caliber gun US equivalent