Japanese
31 cm/50 (12.2") "B" Gun Type 0 (?)
Updated 09 October 2006
Planned for the never-built B-65 class "Super A" cruisers, it appears that no prototypes were ever started.  Information on these weapons is based entirely upon the B-65 preliminary design, completed in September 1940.  The ships were postponed on 6 November 1941 and officially cancelled following the Battle of Midway in June 1942.

Type 0 (Model 1940) is my estimate.

Nomenclature note:  "Japanese Cruisers of the Pacific War" states that:  "Some sources mention 30 cm, but the actual caliber was to have been 310 mm."  This reference further states that 12" (30.5 cm) is "used in non-Japanese publications and non-official Japanese works."
WNJAP_122-50_B-65_painting.jpg

Battlecruiser Design B-65
Detail from a painting by Richard Allison
Scanned from "Battleships:  Axis and Neutral Battleships in World War II"
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Gun Characteristics
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Designation 31 cm/50 (12.2") Type 0 (?) (Model 1940)
Ship Class Used On B-65 class (Warships #795 and 796)
Date Of Design about 1940
Date In Service N/A
Gun Weight N/A
Gun Length oa N/A
Bore Length about 610.2 in (15.500 m)
Rifling Length N/A
Grooves N/A
Lands N/A
Twist N/A
Chamber Volume N/A
Rate Of Fire about 3 rounds per minute
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Ammunition
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Type Bag
Projectile Types and Weights AP - 1,265 lbs. (561 kg)
Bursting Charge N/A
Projectile Length N/A
Propellant Charge N/A
Muzzle Velocity N/A
Working Pressure N/A
Approximate Barrel Life N/A
Ammunition stowage per gun N/A
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Range
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Elevation With 1,265 lbs. (561 kg) AP Shell
Range @ 45 degrees 36,000 yards (32,920 m)
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Mount / Turret Data
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Designation Three-gun Turrets
  B-65 (3)
Weight  N/A
Elevation  -5 / +45 degrees 
Elevation Rate N/A
Train About +150 / -150 degrees
Train Rate N/A
Gun recoil N/A
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Data from
"Naval Weapons of World War Two" by John Campbell
"Japanese Cruisers of the Pacific War" by Eric Lacroix and Linton Wells II
"Battleships:  Axis and Neutral Battleships in World War II" by W.H. Garzke, Jr. and R.O. Dulin, Jr.