United States of America
8"/55 (20.3 cm) Marks 12 and 15
Updated 07 February 2008
Similar in construction to the previous 8"/55 (20.3 cm) Mark 14, but of much lighter construction.

These guns were used in two distinct mountings.  The triple mounting used on the Tuscaloosa (CA-37) sub-class were similar to those used on their sisters of the New Orleans (CA-32) class, but these much lighter guns gave them a significantly lower revolving weight.  The Wichita (CA-45), Baltimore (CA-68) and Oregon City (CA-122) classes had greatly different mountings from previous heavy cruisers.  The guns were individually sleeved and mounted further apart, allowing for better shell dispersion characteristics.  The Baltimore and Oregon City classes also had different shell-handling equipment than did the older cruisers in order to use the new, "super-heavy" 335 lbs. (152 kg) AP projectile.  The older cruisers continued to use the previous 260 lbs. (118 kg) AP projectile.

In 1970, USS St. Paul (CA-73), using Long Range Bombardment Ammunition (LRBA), made some of the longest gun-fire missions in history when she fired on Viet Cong targets some 35 miles away, destroying six structures.

Mark 12 had an autofretted A tube, shrunk on jacket, screw box liner, yoke ring in halves and used a down-swinging Welin breech block.  The Mark 15 was similar but had a smaller chamber and a different rifling twist.  Both Marks had chromium-plated bores.
WNUS_8-55_mk12-15_Saint_Paul_pic.jpg

USS Saint Paul CA-73 about 1966 during shore bombardment off Vietnam
Note that the center guns in each turret are not elevated, showing that these mounts had individually sleeved guns
U.S. Naval Historical Center Photograph # NH 33437
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Click here for additional pictures
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Gun Characteristics
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Designation 8"/55 (20.3 cm) Marks 12 and 15
Ship Class Used On Tuscaloosa (CA-37), Wichita (CA-45), Baltimore (CA-68) and Oregon City (CA-122) classes
Minneapolis (CA-36) was regunned with Mark 15 in 1944
Date Of Design 1933
Date In Service 1939
Gun Weight 17.11 - 17.17 tons (17.38 - 17.45 mt)
Gun Length oa 449.0 in (11.405 m)
Bore Length 440.1 in (11.179 m)
Rifling Length 373.65 in (9.491 m)
Grooves 64 (?)
Lands N/A
Twist Mark 12:  Uniform RH 1 in 35
Mark 15:  Uniform RH 1 in 25
Chamber Volume 4,860 in3 (79.6 dm3)
Rate Of Fire 3 - 4 rounds per minute
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Ammunition
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Type Bag
Projectile Types and Weights
(see Notes 1 and 2)		 AP Mark 21 Mods 1 to 5 (super heavy) - 335 lbs. (152 kg)
AP Mark 19 Mods 1 to 6 - 260 lbs. (118 kg)
SP Common Mark 17 Mods 1 to 4 - 260 lbs. (118 kg)
HC Mark 24 Mods 1 to 5 - 260 lbs. (118 kg)
HC Mark 25 Mod 1 - 260 lbs. (118 kg)

LLRB - 111.8 lbs. (50.7 kg)
Bursting Charge AP Mark 21 - 5.03 lbs. (2.3 kg) Explosive D
AP Mark 19 - 3.64 lbs. (1.7 kg) Explosive D
SP Common - 10.38 lbs. (4.7 kg) Explosive D
HC Mark 24 - 21.34 lbs. (9.7 kg) Explosive D
HC Mark 25 - 21.37 lbs. (9.7 kg) Explosive D

LLRB - 13 lbs. (5.9 kg) PBX-w-106
Projectile Length AP Mark 19 - 36.0 in (91.4 cm)
AP Mark 21 - 36.0 in (91.4 cm)
SP Common - 36.0 in (91.4 cm)
HC Mark 24 - 34.56 in (87.8 cm)
HC Mark 25 - 34.61 in (87.9 cm)

LLRB - 54 in (137.2 cm)
Propellant Charge Full Charge - 85 lbs. (38.6 kg) SPD
Full Flashless Charge - 89 lbs. (40.4 kg) SPCG

Reduced Charge - 55 lbs. (24.9 kg) SPDN
Reduced Flashless Charge - 56 lbs. (25.4 kg) SPDF
Muzzle Velocity Full Charge - New Gun
   AP Mark 21 - 2,500 fps (762 mps)
   AP Mark 19 - 2,700 fps (823 mps)
   SP Common - 2,700 fps (823 mps)
   HC  - 2,700 fps (823 mps)
   LLRB - about 4,000 fps (1,200 mps)

Reduced Charge - New Gun
   AP Mark 21 - 2,000 fps (610 mps)
   AP Mark 19 - 2,220 fps (677 mps)
   SP Common - 2,220 fps (677 mps)
   HC - 2,220 fps (677 mps)
Working Pressure 18.2 tons/in2 (2,870 kg/cm2)
Approximate Barrel Life 715 rounds
Magazine capacity per gun 150 rounds
Notes:

1) AP Mark 21 (super heavy) was apparently issued only to the Baltimore (CA-68) and Oregon City (CA-122) classes.  SP Common had a thin cap and a windscreen.  HC Mark 25 was designed such that it could be forged from the base end.

2) HC projectile bodies could be used with Point Detonating (PD) or Mechanical Time (MT) fuzes.  When used with PD fuzes, they were considered to be HC rounds.  When used with MT fuzes, they were considered to be AAC rounds.  MT fuzes were probably set by hand on the loading trays.

3) Radius of ogive for both AP and HC rounds was 83 inches (211 cm) or about 10crh.  Bourrelet diameter was 7.977 inches (20.3 cm).

4) The propellant charges were in halves.

5) In the late 1960s the "Gunfighter" program at Indian Head Naval Ordnance Station developed Long Range Bombardment Ammunition (LRBA) projectiles.  These were Arrow Shells with a body diameter of 4.125" (10.4 cm) and a fin diameter of 5.0" (12.7 cm) which were sized to be fired from 8" (20.3 cm) guns by using a sabot and obturator system.  Tests with these in 1968 showed maximum ranges of 72,000 yards (66,000 m).  The burster in these shells was PBX-w-106, a castable explosive.  Sabot weighed 17.6 lbs. (8.0 kg) and was discarded as the projectile left the muzzle.  After a test firing off Okinawa of three inert-loaded shells, USS St. Paul (CA-73) in 1970 conducted a two day bombardment mission against Viet Cong positions at ranges up to 70,000 yards (64,000 m).  At the time, St. Paul was the only 8" gunned cruiser still in active service.
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Range
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Elevation
With 335 lbs. (152 kg) AP
Striking Velocity
Angle of Fall
3.1 degrees
6,000 yards (5,490 m)
1,995 fps (608 mps)
3.7
5.8 degrees
10,000 yards (9,140 m)
1,702 fps (519 mps)
7.5
11.3 degrees
16,000 yards (14,630 m)
1,371 fps (418 mps)
16.9
16.5 degrees
20,000 yards (18,290 m)
1,248 fps (380 mps)
25.7
27.5 degrees
26,000 yards (23,770 m)
1,219 fps (372 mps)
41.6
40.7 degrees
30,000 yards (27,430 m)
1,294 fps (394 mps)
54.5
41.0 degrees
30,050 yards (27,480 m)
1,296 fps (395 mps)
54.7
Notes:

1) This table applies only to the Baltimore and Oregon City classes.  The Tuscaloosa and Wichita classes used the 260 lbs. (118 kg) AP round and had the same range characteristics as those listed for the 8"/55 (20.3 cm) Mark 14.

2) Time of flight for 335 lbs. (152 kg) AP Shell with MV = 2,500 fps (762 mps)
   6,000 yards (5,490 m):  8.1 seconds
   10,000 yards (9,140 m): 14.7 seconds
   20,000 yards (18,290 m):  37.2 seconds
   30,000 yards (27,430 m):  77.8 seconds
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Elevation With 260 lbs. (118 kg) HC Shell
Range @ 41.0 degrees 29,800 yards (27,250 m)
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Armor Penetration with 335 lbs. (152 kg) AP Shell
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Range
Side Armor
Deck Armor
10,800 yards (9,880 m)
10.0" (254 mm)
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15,400 yards (14,080 m)
8.0" (203 mm)
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18,400 yards (16,820 m)
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2.0" (51 mm)
20,800 yards (19,020 m)
6.0" (152 mm)
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23,800 yards (21,760 m)
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3.0" (76 mm)
24,400 yards (22,310 m)
5.0" (127 mm)
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27,600 yards (25,240 m)
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4.0" (102 mm)
28,600 yards (26,150 m)
4.0" (102 mm)
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Notes:

1) These figures are taken from armor penetration curves published in 1942.

2) This table applies only to the Baltimore and Oregon City classes.  The Tuscaloosa and Wichita classes used the 260 lbs. (118 kg) AP round and had the same armor penetration characteristics as those listed for the 8"/55 (20.3 cm) Mark 14.
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Mount / Turret Data
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Designation Triple Turrets
   Tuscaloosa (3)

Three-gun Turrets
   Wichita (3), Baltimore (3) and Oregon City (3)
Weight  Tuscaloosa:  250 tons (254 mt)
Wichita:  314 tons (319 mt)
Baltimore and Oregon City Classes:  303 tons (307.7 mt)
Elevation -10 / +41 degrees
Elevation Rate Tuscaloosa class:  About 6 degrees per second
Wichita:  10.1 degrees per second
Baltimore and Oregon City classes:  10.6 degrees per second
Train about +150 / -150 degrees
Train Rate Tuscaloosa class:  About 3 degrees per second
Others:  5.3 degrees per second
Gun recoil 32 in (81 cm)
Loading angle +9 degrees
Notes:

1) To save weight, the barbette design for the Wichita, Baltimore and Oregon City classes was in the shape of a truncated inverted cone.  The barbettes on the Tuscaloosa class may also have been to the same general design.

2) The mountings on the Tuscaloosa class resembled the earlier ones used on the New Orleans (CA-32) class with some dimensional and weight differences.

3) The Wichita and Baltimore classes had a 75 hp training motor.  Each gun had a 15 hp elevation motor, a 15 hp ramming motor and a 10 hp shell hoist motor.  Upper charge hoists on Baltimore and Oregon City were powered by six 2 hp motors and the lower hoists by three 25 hp motors.  All motors drove hydraulic gear.

4) The gun axes on Wichita, Baltimore and Oregon City were about 67 in (170 cm) apart.

5) Each mounting requires a crew of about 66 to operate.

6) USS Boston (CA-69) and USS Canberra (CA-70) traded their stern 8"/55 (20.3 cm) mounting for two twin Terrier missile launchers in the 1950s.  USS Albany (CA-123), USS Chicago (CA-136) and USS Columbus (CA-74) had all of their gun armament removed and were rebuilt into all-missile cruisers.
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Data from
"Naval Weapons of World War Two" by John Campbell
"Powder and Propellants:  Energetic Materials at Indian Head, Maryland, 1890-2001 - Second Edition" by Rodney Carlisle
"US Cruisers:  An Illustrated Design History" and "US Naval Weapons" both by Norman Friedman
"Round Shot to Rockets:  A History of the Washington Navy Yard and the United States Naval Gun Factory" by Taylor Peck
"U.S. Navy Bureau of Ordnance in World War II" by Lt. Cmdr. Buford Rowland, USNR, and Lt. William B. Boyd, USNR
"Cruisers of World War Two" by M.J. Whitley
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"U.S. Explosive Ordnance:  Ordnance Pamphlet 1664 - May 1947" by Department of the Navy
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Special help from Leo Fischer