Description

This gun probably had a longer continuous active service life in the USN than any other large caliber weapon, having first gone to sea aboard USS Wyoming (B-32) in 1912 and remaining in service until the destruction of USS Arkansas (BB-33) during the Bikini tests in 1946. This weapon was only slightly more powerful than the preceding 12"/45 (30.5 cm) Mark 5.

This gun ran up to 19 mods. Mods 0 to 3 were the original guns, all others were lined or relined guns. Most were constructed of liner, A tube, jacket, eight hoops and screw box liner with locking hoops and rings. Mod 3 had no liner, mods 3, 7, 11 and 15 had five rather than eight hoops, mod 5 was never built and mods 11 to 19 had a slightly increased chamber size. All mods used a hand-operated Smith-Asbury breech mechanism. Most mods had side-swinging breech mechanisms, but mods 11 to 14 are listed as being down-swinging.

Actual bore length was 49.5 calibers.

The data that follows is specifically for the 12"/50 Mark 7 Mod 19 which was in service during World War II.

Gun Characteristics

Designation 12"/50 (30.5 cm) Mark 7
Ship Class Used On Wyoming (B-32) class
Date Of Design 1910
Date In Service 1912
Gun Weight 124,140 lbs. (56,310 kg) (including breech)

121,905 lbs. (55,296 kg) (without breech)

Gun Length oa 607.25 in (15.424 m)
Bore Length 594 in (15.087 m)
Rifling Length 500.925 in (12.724 m)
Grooves Mod 0: (72) 0.05 in deep (1.27 mm)
Mods 1, 3 and 19: (72) 0.075 in deep (1.90 mm)
Others: N/A
Lands N/A
Twist Mod 0: Uniform RH 1 in 25
Mods 1, 3 and 19: Increasing RH 1 in 50 to 1 in 32
Others: Uniform RH 1 in 32
Chamber Volume 11,863 cu. in. (194.400 liters)
Rate Of Fire 2 - 3 rounds per minute

Wyoming is listed as having Mark 7 Mod 2 guns and Arkansas Mark 7 Mod 4 and Mark 7 Mod 6 guns at the start of World War II. In January 1945 Arkansas is listed at having Mark 7 Mod 15 to Mark 7 Mod 18 guns.

Ammunition

Type Bag
Projectile Types and Weights
(see Note 1)
AP Mark 15 Mods 1 to 6 - 870 lbs. (394.6 kg)
Common - 870 lbs. (394.6 kg)
HC Mark 16 Mods 1 and 2 - 740 lbs. (335.7 kg)
Bursting Charge AP Mark 15 - 25.0 lbs. (11.3 kg) Explosive D
Common - about 52.2 lbs. (23.7 kg) Explosive D
HC Mark 16 - 58.2 lbs. (26.4 kg) Explosive D
Projectile Length AP Mark 15 - 42.0 in (106.7 cm)
HC Mark 16 - 40.285 in (102.3 cm)
Propellant Charge World War I
   Full Charge - 337 lbs. (152.9 kg) SPD

World War II
   Full Charge - 335 lbs. (152.0 kg) SPD
   Reduced charge - 225 lbs. (102.1 kg) SPD

Muzzle Velocity
(see Note 5)
Full Charge - New Gun
   AP - 2,900 fps (884 mps)
   HC - 3,000 fps (914 mps)

Reduced Charge - New Gun
   AP - 2,100 fps (640 mps)
   HC - 2,125 fps (648 mps)

Working Pressure 17.5 tons/in2 (2,760 kg/cm2)
Approximate Barrel Life 200 rounds
Ammunition stowage per gun 100 rounds
  1. When first brought into service, only AP and Common rounds were provided for these guns. Common was obsolete by 1915 and no longer in production at that time. HC projectiles were introduced in 1942. Besides the HC Mark 16 noted above, there were also HC Mark 21 projectiles, which appear to be identical to the HC Mark 16 but apparently produced by a different manufacturer.
  2. The only armor-piercing projectie still in service during World War II was the AP Mark 15 Mod 6 and this projectile was considered to be obsolete at the time. Although OP 1664 says that this projectile had a large cap and no windshield, Nathan Okun informs me that this is incorrect and that the Mark 15 Mod 6 actually had a small windshield and cap which he describes as follows:
    "About half of the length of the pointed nose above the lower edge of the AP cap is cap (nearly-flat conical top, much like a cone-shaped Johnson AP cap) and the rest is the small, by World War II standards, pointed hollow windscreen (though with a small flat tip). They both form a single circular-arc ogival shape (Secant Ogive - an arc wider than the shell diameter at the hypothetical widest-point lower edge, though of course actually cut off where it meets the surface of the shell's true nose under it - with a 10-caliber radius, I believe), with only the thin crack at the joint of the upper edge of the cap and the lower edge of the windscreen being visible close-up. The rather small AP cap design of the time riding high up on the shell nose looks somewhat like a dunce cap with a flattened tip, since the bottom edge of the shell's true much-blunter pointed nose (2 caliber radius of ogive) under the cap is visible as a bulged out region about another 10% in radius and 10% of the shell length to join the flat side of the body at the upper bourrelet."
  3. Bourrelet diameter was 11.977 inches (30.4 cm).
  4. The propellant was in four bags.
  5. The original muzzle velocity for AP was 2,950 fps (899 mps). This was determined to cause excessive wear and was later reduced to the above figure.

Range

Range of projectiles fired at new gun muzzle velocities
Elevation With AP Shells Striking Velocity Angle of Fall
0.3 degrees 1,000 yards (910 m) 2,802 fps (854 mps) 0.4
1.4 degrees 4,000 yards (3,660 m) 2,521 fps (768 mps) 1.6
10.3 degrees 19,000 yards (17,370 m) 1,524 fps (465 mps) 15.7
11.1 degrees 20,000 yards (18,290 m) 1,489 fps (454 mps) 17.3
12.8 degrees 22,000 yards (20,120 m) 1,432 fps (436 mps) 20.5
14.7 degrees 24,000 yards (21,950 m) 1,392 fps (424 mps) 23.9

Time of flight for AP Shell with MV = 2,900 fps (884 mps)
   4,000 yards (3,660 m): 4.4 seconds
   20,000 yards (18,290 m): 30.1 seconds
   22,000 yards (20,120 m): 34.3 seconds
   24,000 yards (21,950 m): 38.7 seconds

Range of projectiles fired at new gun muzzle velocities
Elevation With HC Shells
15 degrees 23,900 yards (21,850 m)

Armor Penetration with "old-style" AP Shell

Range Side Armor Deck Armor
6,000 yards (5,490 m) 17.4" (442 mm) ---
9,000 yards (8,230 m) 14.7" (373 mm) ---
12,000 yards (10,920 m) 12.3" (312 mm) ---

This data is from "Elements of US Naval Guns" of 1918 and is for AP shells of the World War I era. It is corrected for angle of fall.

Armor Penetration with AP Mark 15 Shell

Range Side Armor Deck Armor
0 yards (0 m) 25.3" (643 mm) ---
14,000 yards (12,802 m) 13.0" (330 mm) 1.5" (38 mm)
17,500 yards (16,002 m) 11.0" (279 mm) 2.0" (51 mm)
20,000 yards (18,290 m) 10.0" (254 mm) ---

These figures are taken from armor penetration curves issued in 1942.

Mount/Turret Data

Designation Two-gun Turrets
   Wyoming (6): Mark 9
Weight 491 tons (498.86 mt)
Elevation -5 / +15 degrees
Elevation Rate N/A
Train about -150 / +150 degrees
Train Rate N/A
Gun Recoil 38 in (96.5 cm)
Loading Angle Any angle

These mounts introduced a single-stage hoist for projectiles, as it was felt that this represented a negligible explosive hazard. Propellant was stowed in magazines on the lower deck and moved up from there to a "passing box" in the turret floor. This box had an upper and lower door, only one of which could be open at a time. Some projectiles were carried on the rotating structure, which was a standard feature of later designs.

Additional Pictures

Sources

"Naval Weapons of World War Two" by John Campbell
"US Naval Weapons" and "US Battleships: An Illustrated Design History" both by Norman Friedman
"A Treatise on Rifling of Guns" by Carl F. Jeansén
"Battleships: United States Battleships, 1935-1992" by W.H. Garzke, Jr. and R.O. Dulin, Jr.
---
"Range and Ballistic Tables 1935" by U.S. Department of Ordnance and Gunnery
"U.S. Explosive Ordnance: Ordnance Pamphlet 1664 - May 1947" by Department of the Navy
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Special help from Leo Fischer and Nathan Okun

Page History

01 March 2008 - Benchmark
15 September 2009 - Added picture of bow turrets
06 December 2010 - Added Nathan Okun comments regarding the AP Mark 15 Mod 6 projectile design
04 May 2015 - Redid photographs of USS Arkansas and USS Wyoming
19 July 2016 - Converted to HTML 5 format