United States of America
12"/50 (30.5 cm) Mark 7
Updated 04 May 2015

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.

WNUS_12-50_mk7_Arkansas_pic.jpg

USS Arkansas B-33 at gunnery practice prior to World War I
Note the "spectators" watching the shoot
U.S. Naval Historical Center Photograph # 63636

WNUS_12-50_mk7_Wyoming_pic.jpg

USS Wyoming BB-32 in March 1930
Note the range dial on the foremast, the rangefinder on Turret II and that several 5"/51 (12.7 cm) hull casemate mounts have been plated over
U.S. Naval Historical Center Photograph # 80-G-466464

WNUS_12-50_mk7_Wyoming_bow_pic.jpg

Bow Turrets on either USS Wyoming (BB-32) or USS Arkansas (BB-33)
Picture is mislabeled as "New York"
Bain News Service Photograph
Library of Congress Photograph ID LC-DIG-ggbain-15880

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Gun Characteristics
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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
Note:  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.
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Ammunition
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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
Notes:

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.

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Range
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Elevation
With 870 lbs. (394.6 kg) AP Shell
MV = 2,900 fps (884 mps)
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
Elevation
With 740 lbs. (335.7 kg) HC Shell
MV = 3,000 fps (914 mps)
   
15 degrees
23,900 yards (21,850 m)
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Note:  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
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Armor Penetration with 870 lb. (394.6 kg) AP Shell
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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)
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Note:  This data is from "Elements of US Naval Guns" of 1918.  It is corrected for angle of fall.
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Armor Penetration with 870 lb. (394.6 kg) Mark 15 AP Shell
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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)
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Note:  These figures are taken from armor penetration curves issued in 1942.
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Mount / Turret Data
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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
Note:  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.
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Data from
"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.
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"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