British
6"/45 (15.2 cm) BL Mark XII and Mark XX
Updated 07 January 2007
First carried by the cruiser HMS Birmingham in 1914, this weapon was used as the secondary gun on the Queen Elizabeth and Royal Sovereign battleship classes and as the main gun on all cruisers built during World War I.  During World War II some AMCs were fitted with guns removed from scrapped ships.

Constructed of inner A tube, A tube, full length wire, full length jacket, breech ring and breech bush which screwed into the A tube.  Used a hand-worked Welin breech-block.  Mark XIIA had a modified chamber with parallel front end, while the Mark XIIB had a modified chamber with a bore of 5.985" (15.202 cm) in an effort to improve accuracy.  Where practical, it was preferred not to mix Mark XIIB with Mark XII or XIIA, but all guns were interchangeable.  A total of 463 guns were manufactured, of which 431 remained in the Navy as of 1939.

 Actual bore length was 45.11 calibers.

The Mark XX was similar to the Mark XII but had a chamber that was shorter and lower in volume.  This design was an attempt to fix an oval wear problem found with the Mark XII.  Thirty-seven guns were ordered in 1917, but the order was cancelled after trials showed that accuracy suffered.  The only Mark XX actually completed was converted back to the Mark XII standard.
WNBR_6-45_mk12_pic.jpg

Loading practice with a 6"/45 (15.2 cm) Mark XII aboard a C class cruiser in 1935
MoD photograph
WNBR_6-45_mk12_Diomede_pic.jpg

HMS Diomede
"Weatherproof" Mark XVI mounting on bow and open-back Mark CPXIV mountings in the other positions
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Images at The Vickers Photographic Archive

Test results of 6" (15.2 cm) CPBC against 1" (2.254 cm) armor:  4645
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Gun Characteristics
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Designation 6"/45 (15.2 cm) BL Mark XII
Ship Class Used On Queen Elizabeth, Royal Sovereign, Arethusa, "B", "C", "D" and "E" cruiser classes
Monitors M.29 to M.33, Abercrombie, Raglan and Picton
Date Of Design 1913
Date In Service 1914
Gun Weight 15,428 lbs. (6,998 kg)
Gun Length oa 279.7 in (7.105 m)
Bore Length 270.0 in (6.858 m)
Rifling Length 230.6 in (5.856 m)
Grooves (36) 0.0535 in deep x 0.3759 (1.36 x 9.548 mm)
Lands 0.1477 in (3.752 mm)
Twist Uniform RH 1 in 30
Chamber Volume 1,770 in3 (29.0  dm3)
Rate Of Fire
(see Note)		 5 - 7 rounds per minute
Note:  The Rate of Fire figure given above is found in references for British guns of this caliber, but "Warrior to Dreadnought:  Warship Development 1860-1905" quotes Jellicoe's 1906 figures for rates of fire for these guns in gunlayers' tests and in battle practice and notes that the latter figures corresponded well to those actually attained by the Japanese at Tsushima:

Gunlayers Test:  12 rounds per minute
Battle Practice:  4 rounds per minute

In "Jutland:  An Analysis of the Fighting" by John Campbell, it is stated that almost all British capital ships had few or slow hoists for their 6" (15.2 cm) guns and that once the ready ammunition was used up, the rate of fire dropped to about 3 rounds per minute.  For light cruisers the rate of supply was was about three to five rounds per minute per gun, and usually closer to the lower figure.
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Ammunition
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Type Bag
Projectile Types and Weights CPBC 2crh - 100 lbs. (45.36 kg)
CPC 4crh - 100 lbs. (45.3 kg)
HE 4crh - 100 lbs. (45.3 kg)
HE 6crh - 112 lbs. (50.8 kg)
Bursting Charge CPC - 7.5 lbs. (3.4 kg)
HE 4crh - 13.3 lbs. (6.0 kg)
Projectile Length CPC - 23.5 in (59.7 cm)
HE - 22.9 in (58.2 cm)
Propellant Charge Standard Charge:  27 lbs. 2 oz. (12.3 kg) SC 122
Super Charge:  34.25 lbs. (15.54 kg) SC 150
Muzzle Velocity Mark XII and XIIA:  2,800 fps (853 mps)
Mark XII B:  2,825 fps (861 mps)
Mark XII with super charges:  3,070 fps (936 mps)
Working Pressure 20 tons/in2 (3,150 kg/cm2)
Approximate Barrel Life 670 rounds
Ammunition stowage per gun Queen Elizabeth and Royal Sovereign classes:  130 rounds plus 100 star shell per ship
Arethusa, "B" and early "C" classes:  150 rounds
Later "C" and "D" classes:  200 rounds
"E" class:  240 rounds for forward guns, 215 rounds for remainder
Monitors:  250 rounds
Notes: 

1) The propellant was in a single bag.  Flashless does not seem to have been issued.

2) Super charges were issued only to Aphis and AMC Canton.  These ships fired 6crh HE and CPBC in Canton.  Outfits for other ships were 4crh shells, CPBC and HE, or CPC and HE in battleships and AMC.

3) The outfit per gun for monitors was 100 CPC, 125 HE and 25 shrapnel.
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Range
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Elevation Mark XIIB with 4crh 100 lbs. (45.36) CPBC Shell
Range @ 15 degrees 13,500 yards (12,344 m)
Range @ 20 degrees 15,660 yards (14,320 m)
Range @ 30 degrees 18,750 yards (17,145 m)
Range @ 40 degrees 21,500 yards (19,660 m)
Note:  The Mark XII and XIIA would have had ranges about 40 to 50 yards (37 to 46 m) less than the figures above.
Elevation Mark XII with 112 lbs. (50.8 kg) HE 6crh shells and super charges
Range @ 20 degrees 20,020 yards (18,310 m)
Range @ 30 degrees 23,770 yards (21,735 m)
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Armor Penetration with 100 lbs. (45.3 kg) CPC Shell
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Range KC Side Armor
7,500 yards (6,860 m) 3.5 in (8.9 cm)
Note:  Data from "Big Gun Monitors."
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Mount / Turret Data
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Designation Single mounts
   Queen Elizabeth (14) and Royal Sovereign (14):  PIX in casemate mountings
   Arethusa (2):  PVII and PVII*
   "B" class (4):  PVII
   "C" class (5):  CPXIII, CPXIII* and C XIII**
   "D" class (6), Emerald (7) and Enterprise (5):  CPXIV
   Diomede (1):  Mark XVI
   Monitors (2):  PIX
   AMCs:  PVII and PVII*
 

Twin Mount
   Enterprise (1):  Mark XVII
Weight (less shield) PVII:  14.13 tons (14.35 mt)

PVII*:  13.90 tons (14.12 mt)

PIX:  13.375 tons (13.59 mt)

CPXIII, XIII* and XIII**:  14.5 - 14.8 tons (14.7 - 15.1 mt)

CPXIV:  14.82 tons (15.06 mt)

Mark XVI and Mark XVII:  N/A
Elevation
(see Notes 1 and 2)		 PVII and PVII*:  -7 / +15 degrees
   Some later modified to +20 degrees

PIX:  -7 / +14 degrees
   Some later modified to +17.5 degrees

CPXIII, XIII* and XIII**:  -7 / +30 degrees

CPXIV:  -5 / +40 degrees

Mark XVI:  -5 / +40 degrees

Mark XVII:  -5 / +40 degrees
Elevation Rate Manual operation, only
Train about +120 / -120 degrees
Train Rate Manual operation, only
Gun recoil N/A
Notes:

1) PVII and PIX were pedestal mounts while CPXIII and CPXIV were central pivot mountings.  The latter were easier to hand-operate.  As the central pivot mountings supported the gun on side-brackets rather than a central pedestal, it was easier to provide for high elevation.

2) Guns in casemates were generally limited to +14 degrees.

3) The four guns alongside "Y" turret on the Queen Elizabeth class were removed almost immediately as they were washed out in any kind of seaway.  Two of these were then resited on the forecastle deck.

4) Those Queen Elizabeth class ships modernized during the 1930s had some or all of these guns removed.  The Royal Sovereign class had two guns removed during refits in the 1930s.

5) The Mark XVI on Diomede was a weatherproof mounting in Number 1 position.  This mounting provided a motor-driven shell hoist in place of the hoists and whips used on other cruisers.  The mounting was considered a success and was generally recommended for future construction but no more were built.

6) The Mark XVII twin mounting on Enterprise proved very successful and was further developed into the twin 6"/50 (15.2 cm) mountings used on the Leander and Arethusa class cruisers.  This design was also used as a prototype for the twin 6"/50 (15.2 cm) mountings used on the Nelson class battleships, with the major difference being that the battleship mountings used a shorter trunk than did the cruiser mountings.
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Data from
"Warrior to Dreadnought:  Warship Development 1860-1905" by D.K. Brown
"Big Gun Monitors:  The History of the Design, Construction and Operation of the Royal Navy's Monitors" by Ian Buxton
"Jutland:  An Analysis of the Fighting", "Naval Weapons of World War Two" and "British Naval Guns 1880-1945 No 12" article in "Warship Volume VIII" all by John Campbell
"British Battleships of World War Two" and "British Cruisers of World War Two" both by Alan Raven and John Roberts
"A Concentrated Effort:  Royal Navy Gunnery Exercises at the End of the Great War" article by William Schleihauf in "Warship International" No. 2, 1998
"Cruisers of World War Two" by M.J. Whitley
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"History of the Gun" by W.L. Ruffell as reproduced on New Zealand Permanent Force Old Comrades' Association Website