Low-power weapon used as AA and ASW weapon on larger Japanese merchant ships. Also used in coastal defense batteries in the Marianas.

Constructed of monobloc barrel with a screw breech.

Actual bore diameter was 20.32 cm (8.0").

Gun Characteristics

Designation 20 cm/12 (8") Short Gun
Ship Class Used On Merchant ships over 5,000 grt
Date Of Design 1943
Date In Service 1943
Gun Weight 1,390 lbs. (630 kg)
Gun Length oa 99.21 in (2.520 m)
Bore Length 96.0 in (2.438 m)
Rifling Length 78.1 in (1.984 m)
Grooves (32) 0.059 in deep x 0.654 in (1.5 mm x 16.61 mm)
Lands 0.131 in (3.34 mm)
Twist Increasing RH 1 in 30 to 1 in 13
Chamber Volume 891 in3 (14.6 dm3)
Rate Of Fire 1 4.5 rounds per minute
  • ^ROF from "Naval Weapons of World War Two" but as the mountings for these guns were hand-trained and elevated and needed to return to a low angle for loading, I believe the rate of fire to be overstated.


Type Separate
Projectile Types and Weights 1a HE: 103.6 lbs. (47 kg)
IS: 103.6 lbs. (47 kg)
Bursting Charge 30.0 lbs. (13.6 kg)
Projectile Length 22.4 in (56.9 cm)
Propellant Charge 4.41 lbs. (2 kg)
Muzzle Velocity 1,001 fps (305 mps)
Working Pressure 4.1 to 4.4 tons/in2 (650 to 700 kg/cm2)
Approximate Barrel Life 2,000 rounds
Ammunition stowage per gun N/A
  • ^Two kinds of incendiary shrapnel (IS) were supplied.


Range with 103.6 lbs. (47 kg) AP
Elevation Distance
Maximum range 6,890 yards (6,300 m)
75 degrees 10,830 feet (3,300 m)

Mount/Turret Data

Designation Single Mountings
Weight 4 tons (4.1 mt)
Elevation -15 / +75 degrees
Elevation Rate 8 degrees per second (manual operation)
Train 360 degrees (?)
Train Rate 8 degrees per second (manual operation)
Gun recoil N/A
Loading angle about 10 degrees

These mountings are described as being heavy and slow, which may mean that the firing rates are overstated, especially at high elevations. Ammunition supply relied upon the ships' cargo hoists and simple derricks.

Additional Pictures


Data from:

  • "Naval Weapons of World War Two" by John Campbell


  • US Naval Technical Mission to Japan report O-54(N): Japanese Naval Guns