Japan

20 cm/12 (8") Short Gun

Click for larger image. — 20 cm/12 (8") Short gun used as coastal defense battery. Note the projectiles and cartridge case. U.S. Navy Photograph.

Description

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.

Ammunition

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

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 ^1b
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

Click for larger image. — 20 cm/12 (8") Short gun used as coastal defense battery at Bangi point, Agat Beachhead, Guam in July 1944. USMC Photograph # 91661.

Click for larger image. — One of two 20 cm/12 (8") Short guns captured at Bangi Point, Guam. Note the relatively thin barrel walls. Some guns had a reinforcing tube bolted around the barrel to strengthen it. Picture taken on 05 October 1944. National Archive Photograph No. 80-G-247626. National Park Service WAPA Photograph No. 34 BangiCoastal.jpg.

Click for larger image. — Ammunition for above 20 cm/12 (8") Short gun. National Archive Photograph No. 80-G-247630. National Park Service WAPA Photograph No. 37 BangiAmmo.jpg.

Click for larger image. — The other 20 cm/12 (8") Short gun captured at Bangi Point, Guam. National Archive Photograph No. 80-G-247631. National Park Service WAPA Photograph No. 38 BangiCD.jpg.

Click for larger image. — Overhead view of breech. Note the shell loading tray. National Archive Photograph No. 80-G-247629.

Click for larger image. — Dismounted 20 cm/12 (8") Short gun captured on Guam. National Archive Photograph No. 80-G-281180. National Park Service WAPA Photograph No. 14 20cm Jgun.jpg.

Sources

"Naval Weapons of World War Two" by John Campbell
US Naval Technical Mission to Japan report O-54(N): Japanese Naval Guns

External Resources

Japanese 12 cm and 20 cm Short guns - Dirk Spennemann
Japanese 20 cm Short guns at Saipan - Dirk Spennemann

Page History

19 December 2016 - Benchmark
15 January 2021 - Converted to HTML 5 format
24 September 2022 - Updated images added External Resources