Japan
12 cm/45 (4.7") 10th Year Type
Updated 26 December 2011
This was an AAA weapon derived from the single-purpose 12 cm/45 (4.7") 3rd Year Type.  Most cruisers built in the 1920s and early 1930s were armed with this gun for their secondary batteries.  During World War II it was used in coastal batteries and still used afloat on unreconstructed cruisers and on many escort and auxiliary ships as well as the carrier Akagi.  Considered to be very reliable weapons.

The four Myôkô class cruisers conducted AA trials with these guns in 1931.  Steaming at 18 knots and using the the Type 89 HA computer, the cruisers scored 2.2% hits against aerial targets towed at 60 to 70 knots at altitudes of 1,500 to 2,000 m (4,900 to  6,600 feet).  Firing was at an average of 6.4 rounds per minute at ranges between 2,000 and 5,500 m (2,200 and 6,000 yards).

The carrier Kaga and those cruisers modernized during the late 1930s had these weapons replaced by the 12.7 cm/40 (5") Type 89 AA gun.  However, Akagi carried them to her end at Midway.

The earlier guns were of built-up construction but the later ones were of monobloc construction.  All used a semi-automatic sliding breech-block mechanism.  A total of about 3,000 guns were manufactured, with 2,320 (one source says 2,152) of them being produced between 1942 to 1945.
WNJAP_47-45_10ns.jpg

12 cm/45 (4.7") guns on Heavy Cruiser IJN Nachi
(IWM picture via CPL)
WNJAP_47-45_10ns_Guam_pic.jpg

12 cm/45 (4.7") gun captured on hill east of the Orote Peninsula Airfield, Guam
Picture taken on 5 October 1944
National Archive Photograph No. 80-G-247827
National Park Service WAPA Photograph No. 11 1 of 4gun.jpg
WNJAP_47-45_10ns_breech_pic.jpg

Breech of above weapon
National Archive Photograph No. 80-G-247826
National Park Service WAPA Photograph No. 10 Orte4guns.jpg
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Gun Characteristics
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Designation 12 cm/45 (4.7") 10th Year Type (Model 1921)
Ship Class Used On Originally:  Most cruisers
World War II:  Akagi, Taiyo, Unyo, Kako, Aoba, Myôkô, Chokai, Maya, Yubari, Ioshima classes as well as many smaller escort and auxiliary vessels
Date Of Design 1921
Date In Service 1926
Gun Weight 2.9 tons (2.95 mt)
Gun Length oa 220.6 in (5.604 m)
Bore Length 212.6 in (5.400 m)
Rifling Length 183 in (4.649 m)
Grooves
(see Note)		 (34) 0.057 in deep x 0.263 in (1.45 mm x 6.688 mm)
Lands 0.173 in (4.40 mm)
Twist Uniform RH 1 in 28
Chamber Volume 657.5 in3 (10.774 dm3)
Rate Of Fire 10 - 11 rounds per minute maximum
6 - 8 rounds per minute effective
Note:  Some guns had 36 instead of 34 grooves.
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Ammunition
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Type Fixed
Weight of Complete Round Common Type 0 HE - 75 lbs. (34 kg)
Common Type 1 HE - 75 lbs. (34 kg)

Others - N/A
Projectile Types and Weights
(see Note 1)		 Common Type 0 HE - 44.9 lbs. (20.3 kg)
Common Type 1 HE - 44.9 lbs. (20.3 kg)
Common Type 4 IS - N/A
ASW - 36.3 lbs. (16.4 kg)
Illum - about 44.9 lbs. (20.3 kg)

New Type Projectile - 49.6 lbs. (22.5 kg)
Bursting Charge Common Type 0 HE - 3.75 lbs. (1.7 kg)
Common Type 1 HE - 4.07 lbs. (1.9 kg)
ASW - 7.19 lbs. (3.8 kg)

New Type Projectile - 4.07 lbs. (1.9 kg)
Projectile Length Common Type 0 HE - 16.0 in (40.8 cm)
Common Type 1 HE - 16.0 in (40.8 cm)
ASW - 16.4 in (41.5 cm)
New Type Projectile - 20.3 in (51.6 cm)

Complete Common HE Rounds - 37.0 in (94 cm)
Propellant Charge 12.1 lbs. (5.5 kg) 30 DC
Cartridge - 30 lbs. (14.4 kg)
Muzzle Velocity Common HE - 2,707 to 2,723 fps (825 to 830 mps)
Illum - 2,297 fps (700 fps)
ASW - 820 fps (250 mps)
Working Pressure 16.8 tons/in2 (2,650 kg/cm2)
Approximate Barrel Life 700 - 1,000 rounds
Ammunition stowage per gun 200 - 250 rounds
Notes:

1) IS is my abbreviation for the incendiary shrapnel round (sankaidan) intended for AA use.

2) A serious fault of this weapon was that the propellant did not totally combust before the projectile left the muzzle, leading to irregular muzzle velocities.  Late in the war, a tubular grain charge was introduced which gave much more regular performance without increasing the working pressure or muzzle velocity.

3) The flat-nosed ASW projectile was issued in 1943 following extensive testing.

4) Illumination rounds were rated at 600,000 candle power and had a maximum range of 15,310 yards (14,000 m).

5) "New Type Projectile" is the designation used in the US Naval Technical Mission to Japan report O-19.  This projectile was under development at the end of the war and production was started but it does not appear to have entered actual service use.  This projectile was boat-tailed, more streamlined at 13crh, had a maximum surface range of 22,430 yards (20,500 m) and a maximum AA Ceiling of 42,660 feet (13,000 m) at 75 degrees elevation.
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Range
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Elevation With 45.0 lbs. (20.4 kg) HE Shell and 30 DC propellant
Range @ 45 degrees 17,500 yards (16,000 m)
AA Ceiling @ 75 degrees 32,800 feet (10,000 m)
Elevation With 36.3 lbs. (16.4 kg) ASW Shell
Range @ 40 degrees 4,375 yards (4,000 m)
Note:  Minimum range of ASW shell is given as 750 yards (820 m).  Ranges less than this tended to ricochet.
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Mount / Turret Data
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Designation Single Mounts
   Aoba (4), Myôkô (6), Yaeyama (2) and Shirataka (3):  B
   Furutaka (4), Takao (4), Taiyo (4) and "improved Taiyo" (8):  B2
   Mikura (Type B escort) (3), Ukuru (Modified Type B escort) (3), Kaikoban I (Type C escort) (2) and Kaikoban II (Type D escort) (2):  C

Twin Mounts
   Akagi (6), Kaga (6), small escorts and auxiliaries:  A2
Weight  Single Mounts
   B:  7.7 tons (7.8 mt)
   B2:  9.8 tons (10.0 mt)
   C:  N/A

Twin Mounts
   A2:  20 tons (20.3 mt)
Elevation -10 / +75 degrees 
Elevation Rate B:  Manually operated, only

B2 and A2:  6.5 degrees per second

C:  N/A (probably 6.5 degrees per second)
Train Larger ships:  About +/- 70 degrees 
Train Rate B:  Manually operated, only

B2 and A2:  10 degrees per second

C:  N/A (probably 10 degrees per second)
Gun recoil N/A
Loading Angle Any
Notes:

1) The Aoba and Myôkô classes were refitted with B2 mounts during the early 1930s.

2) C, B2 and A2 mounts were electro-hydraulically powered.

3) Information about the Type C mounting is a result of my deductions from data presented in the sources below.
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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
"Anatomy of the Ship:  The Heavy Cruiser Takao" by Janusz Skulksi
"Japanese Warships of World War II" by A.J. Watts
"Cruisers of World War Two" by M.J. Whitley
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US Naval Technical Mission to Japan report O-19:  Japanese Projectiles General Types
US Naval Technical Mission to Japan report O-47(N)-1:  Japanese Naval Guns and Mounts-Article 1, Mounts Under 18"
Page History

28 December 2008 - Benchmark
25 December 2011 - Added information about cruiser and escort mountings