20 cm/50 3rd Year Type naval gun
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 Kaga and the Akagi before their 1935 reconstruction. 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 and Kaga carried only six, divided in three casemates per side, after the removal of the four guns in two turrets on both ships placed on the second deck.
Tone-class cruiser Chikuma mounted all 4 turrets forward of the bridge.
Starboard quarter of Kaga with three casemate guns visible.
A heavy cruiser was a type of cruiser, a naval warship designed for long range and high speed, armed generally with naval guns of roughly 203 mm (8 inches) in calibre, whose design parameters were dictated by the Washington Naval Treaty of 1922 and the London Naval Treaty of 1930. Heavy cruisers were generally larger, more heavily-armed and more heavily-armoured than light cruisers while being smaller, faster, and more lightly-armed and armoured than battlecruisers and battleships. Heavy cruisers were assigned a variety of roles ranging from commerce raiding to serving as 'cruiser-killers,' i.e. hunting and destroying similarly-sized ships
HMS Frobisher, a Hawkins-class cruiser around which the Washington Naval Treaty limits for heavy cruisers were written.
Armoured cruiser USS Tennessee, armed with four 10-inch (250 mm) guns, ca. 1907
HMS Hawkins, lead ship of her class.
HMAS Canberra, a County-class "treaty cruiser".