20 cm / 50 3rd Year Type naval gun These were built - up gun s 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 powder charge to fire projectiles weighing at a velocity of . Useful life was 300 effective full charge s ( EFC ) per gun . These guns were initially installed in type A low - angle ( 25° ) single mounts aboard Furutaka - class cruiser s , 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 powder charge to fire projectiles weighing at a velocity of . 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 cruiser s . 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 casemate s with a maximum elevation of 25 degrees limiting maximum range to . 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 ) . Gun mounts . 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 .