United States of America
12"/50 (30.5 cm) Mark 8
Updated 24 August 2014

This gun was a major improvement over the 12"/50 (30.5 cm) Mark 7 guns used on the USS Wyoming (BB-33) class and was of a simpler, lighter construction.  Designed to fire the new "super-heavy" AP projectiles, their side belt armor penetration at 20,000 to 30,000 yards (18,290 to 27,430 m) was almost identical to and the deck plate penetration better than the larger 14"/50 (35.6 cm) guns used on U.S. pre-treaty battleships.

The manufacturing of these guns shows the changes brought about by the massive production increases caused by World War II.  While the 16"/45 (40.6 cm) guns and turrets for the battleship North Carolina and her sister were made almost entirely at the Naval Gun Factory in Washington D.C., the rapid acceleration in shipbuilding during the war forced the Navy to move much of the production of heavy guns to other manufacturers.  As a result, the forgings for the 12"/50 (30.5 cm) Mark 8 guns were made by Midvale and Bethlehem Steel Corporations and then sent to the Naval Gun Factory for processing.  The Watervliet Arsenal also worked on the guns until they were approximately 65 percent complete and then sent them back to the Naval Gun Factory where the guns were finished.  The turrets for these ships were built at the Naval Gun Factory.

Constructed of liner, jacket, three hoops, two locking rings and a screw box liner.  The bore was chromium plated.  Used a down-swinging hydraulically operated Welin breech.  The first prototype gun was completed in January 1942.

WNUS_12-50_mk8_Guam_pic.jpg

USS Guam CB-2 at Pearl Harbor in February 1945
Note that the 12" (30.5 cm) guns are individually sleeved, the bow 5" (12.7 cm) mount is superfiring and that the port catapult is swung out underneath the aircraft crane
U.S. Naval Historical Center Photograph # NH 97130

WNUS_12-50_mk8_Hawaii_pic.jpg

Bow of unfinished USS Hawaii CB-3 in January 1947
Note the periscopes on Turret II
U.S. Naval Historical Center Photograph # NH 93586

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Gun Characteristics
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Designation 12"/50 (30.5 cm) Mark 8
Ship Class Used On Alaska (CB-1) class
Date Of Design 1939
Date In Service 1944
Gun Weight 121,856 lbs. (55,262 kg) (including breech) 
108,800 lbs. (49,351 kg) (without breech)
Gun Length 612 in (15.545 m)
Bore Length 600 in (15.240 m)
Rifling Length 511.96 in (13.004 m)
Grooves 72
Lands N/A
Twist Uniform RH 1 in 25
Chamber Volume 11,863 in3 (194.4 dm3)
Rate Of Fire 2.4 - 3.0 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 18 Mod 1 - 1,140 lbs. (517.093 kg)
HC Mark 17 Mods 1 and 2 - 940 lbs. (426.38 kg)
Bursting Charge AP Mark 18 - 17.4 lbs. (7.9 kg) Explosive D
HC Mark 17 - 79.44 lbs. (36.0 kg) Explosive D
Projectile Length AP Mark 18 - 54.00 in (137.2 cm)
HC Mark 17 - 50.18 in (127.5 cm)
Propellant Charge Full Charge - 275.00 lbs. (124.7 kg) SPD

Special Charge - 245.00 lbs. (111.1 kg) SPD

Reduced Charge - 135.00 lbs. (61.2 kg) SPD
Reduced Flashless Charge - 135 lbs. (61.2 kg) SPDF

Muzzle Velocity Full Charge - New Gun
   AP - 2,500 fps (762 mps)
   HC - 2,650 fps (808 mps)

Special Charge - New Gun
   AP - 2,300 fps (701 mps)
   HC - 2,435 fps (742 mps)

Reduced Charge - New Gun
   AP - 1,800 fps (549 mps)
   HC - 1,965 fps (599 mps)

Working Pressure 18.0 tons/in2 (2,835 kg/cm2)
Approximate Barrel Life 344 rounds
Ammunition stowage per gun 166 rounds
Notes:

1) AP projectiles had the windshield threaded to the AP cap and held in place by five equally spaced notches which were staked.  The AP cap weighed 110.98 lbs. (50.3 kg) and was soldered to the nose.  In addition to the solder, the cap was also secured with seven crimp caps equally spaced around the periphery of the nose.

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.  There was also HC Mark 22, which was the same as HC Mark 16 but apparently produced by a different manufacturer.

3) Propellant was in four bags.

4) Bourrelet diameter was 11.977 inches (30.4 cm).

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Range
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Elevation
1,140 lbs. (517.093 kg)
AP Mark 18 shell
(full charge)
1,140 lbs. (517.093 kg)
AP Mark 18 shell
(reduced charge)
940 lbs. (426.38 kg)
HC Mark 17/22 shell
(full charge)
10 degrees
16,700 yards (15,270 m)
9,500 yards (8,687 m)
17,000 yards (15,545 m)
15 degrees
22,200 yards (20,300 m)
12,950 yards (11,841 m)
22,200 yards (20,300 m)
20 degrees
26,600 yards (24,323 m)
15,850 yards (14,493 m)
26,500 yards (24,232 m)
25 degrees
30,450 yards (27,843 m)
18,000 yards (16,459 m)
30,100 yards (27,523 m)
30 degrees
33,600 yards (30,724 m)
19,850 yards (18,151 m)
33,200 yards (30,358 m)
35 degrees
36,000 yards (32,918 m)
21,200 yards (18,151 m)
35,550 yards (32,507 m)
40 degrees
37,800 yards (34,564 m)
22,100 yards (20,208 m)
37,250 yards (34,061 m)
45 degrees
38,573 yards (35,271 m)
22,604 yards (20,669 m)
38,021 yards (34,766 m)
Note:  Time of flight for AP Shell with MV = 2,500 fps (762 mps)
   10,000 yards (9,140 m): 13.6 seconds
   20,000 yards (18,290 m):  31.5 seconds
   30,000 yards (27,430 m):  55.5 seconds
   36,000 yards (32,920 m):  75.6 seconds
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Armor Penetration using 1,140 lbs. (517.093 kg) AP Mark 18 Shell
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Range
Side Armor
Deck Armor
Striking Velocity
Angle of Fall
0 yards (0 m)
24.48" (622 mm)
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2,500 fps (762 mps)
0.0
5,000 yards (4,572 m)
21.34" (542 mm)
0.51" (13 mm)
2,215 fps (675 mps)
2.6
10,000 yards (9,144 m)
18.23" (463 mm)
1.26" (32 mm)
1,948 fps (594 mps)
6.0
15,000 yards (13,716 m)
15.56" (395 mm)
2.14" (54 mm)
1,745 fps (532 mps)
11.0
20,000 yards (18,288 m)
12.73" (323 mm)
3.02" (77 mm)
1,550 fps (472 mps)
17.5
25,000 yards (22,860 m)
10.52" (267 mm)
4.02" (102 mm)
1,435 fps (437 mps)
25.3
30,000 yards (27,432 m)
9.08" (231 mm)
5.11" (130 mm)
1,400 fps (427 mps)
32.8
35,000 yards (32,004 m)
7.35" (187 mm)
7.18" (182 mm)
1,427 fps (437 mps)
44.5
Note:  The above information is from "Battleships:  United States Battleships 1935-1992" by Garzke and Dulin and is based upon the USN Empirical Formula for Armor Penetration.  These values are in substantial agreement with armor penetration curves published in 1942.
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Mount / Turret Data
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Designation Three-gun Turrets
   Alaska (3)
Weight 922 - 934 tons (937 - 949 mt)
Elevation -3 / +45 degrees
Rate of Elevation 12 degrees per second
Train -150 / +150 degrees
Rate of Train 5 degrees per second
Gun Recoil 36 in (91 cm)
Loading Angle +7 degrees
Notes:

1) Although similar to the triple 16 inch (40.6 cm) turrets built for battleships, this mount had several features not found on other USN large-caliber guns of the 1930s and 1940s.  It had a two-stage powder hoist vs. a single stage in the 16" (40.6 cm) mounts of the battleships.  This change increased the rate of fire and provided a greater degree of safety.  A unique feature was the provision of a projectile rammer to transfer projectiles from the projectile storage room onto the rotating ring.  However, this feature was not entirely satisfactory and it was omitted from the Hawaii and removed from the upper projectile flats on the Alaska and Guam.

2) These mounts had numerous problems, mainly because they had been rushed into design and construction without being properly tested.  BuOrd noted that "this turret is an outstanding example of a case where prior testing of a new design by the manufacture of a pilot model was required."  It appears that these problems were satisfactorily resolved in service, as former crewmembers have informed me that there were no major problems during their war-cruises.

3) Turrets were electrically powered through hydraulic gear.  Training motor was 150 hp and each gun had a 35 hp motor for elevation and a 40 hp ramming motor.  The shells were stowed vertically on two decks and transferred to shell rings similar to the ones used on the 16" (40.6 cm) battleships.  These rings were driven by 40 hp motors.  Shells were carried by pusher hoists powered by 50 hp motors.  Charges were transferred by ram type hoists, 35 hp lower hoist and 25 hp upper hoist.  Turret II had a third 40 hp shell ring at the base of the turret which supplied axial lower hoists driven from two 50 hp motors.

4) The gun axes were about 98 in (249 cm) apart.

5) Ten gun turrets including one intended as a spare were built during the war.  These turrets cost about $1,550,000 each, not including the cost of the guns themselves, making them the USN's most expensive gun mounting of World War II.

6) Armor thickness for the Alaska class (CB-1) as given in "Battleships:  United States Battleships, 1935 - 1992" by W.H. Garzke, Jr. and R.O. Dulin, Jr.:
    Face:  12.8 in (32.5 cm)
    Front Sides:  6.0 in (15.2 cm)
    Rear Sides:  5.25 in (13.3 cm)
    Rear:  5.25 in (13.3 cm)
    Roof:  5.0 in (12.7 cm)

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Data from
"Naval Weapons of World War Two" by John Campbell
"US Cruisers:  An Illustrated Design History" and "US Naval Weapons" both by Norman Friedman
"Battleships:  United States Battleships, 1935-1992" by W.H. Garzke, Jr. and R.O. Dulin, Jr.
"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
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"U.S. Explosive Ordnance:  Ordnance Pamphlet 1664 - May 1947" by Department of the Navy
Page History

07 February 2008 - Benchmark
24 August 2014 - Added turret armor note