Third year type 20 cm/50 caliber guns 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 carried six. 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.