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Designed exclusively for the Alaska class large cruisers, this gun was a major improvement over the 12" (30.5 cm) Mark 7 guns used on the USS Wyoming BB-33 class. Designed to fire the new "super-heavy" APC 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) caliber used on U.S. pre-treaty battleships. 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. |
| Gun Characteristics | |
| Designation | 12"/50 (30.5 cm) caliber Mark 8 |
| Ship Class Used On | Alaska 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) |
| Number Of Grooves | 72 |
| Length Of Rifling | 511.96 in (13.004 m) |
| Chamber Volume | 11,863 cu. in. (194.4 liters) |
| Rate Of Fire | 2.4 - 3.0 rounds per minute |
| Ammunition | |
| Type | Bag |
| Projectile Types and Weights | AP - 1,140 lbs. (517.093 kg) HC - 940 lbs. (426.38 kg) |
| Bursting Charge | AP - about 17 lbs. (7.7 kg) HC - about 75 lbs. (34.1 kg) |
| Projectile Length | about 54 in (137.2 cm) |
| Propellant Charge | Full charge - 272.65 lbs (123.672 kg) NC Reduced charge - 134.65 lbs (61.072 kg) NC |
| Muzzle Velocity | AP Full charge: 2,500 fps (762 mps) Reduced charge: 1,800 fps (549 mps) HC |
| Working Pressure | 18 tons/in2 (2,835 kg/cm2) |
| Approximate Barrel Life | 344 rounds |
| Magazine capacity per gun | 166 rounds |
| Note: Propellant was in four bags. | |
| Range | |||
| Elevation | AP Mark 18 shell (full charge) 1,140 lbs. (517.093 kg) | AP Mark 18 shell (reduced charge) 1,140 lbs. (517.093 kg) | HC Mark 17/22 shell (full charge) 940 lbs. (426.38 kg) |
| 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) |
| Armor Penetration using 1,140 lbs. (517.093 kg) AP Mark 18 Shell | ||||
| Range | Side Armor | Deck Armor | Striking Velocity | Angle of Fall |
| 0 yards (0 m) | 24.48" (622mm) | --- | 2,500 fps (762 mps) | 0.0 |
| 5,000 yards (4,572 m) | 21.34" (542mm) | 0.51" (13mm) | 2,215 fps (675 mps) | 2.6 |
| 10,000 yards (9,144 m) | 18.23" (463mm) | 1.26" (32mm) | 1,948 fps (594 mps) | 6.0 |
| 15,000 yards (13,716 m) | 15.56" (395mm) | 2.14" (54mm) | 1,745 fps (532 mps) | 11.0 |
| 20,000 yards (18,288 m) | 12.73" (323mm) | 3.02" (77mm) | 1,550 fps (472 mps) | 17.5 |
| 25,000 yards (22,860 m) | 10.52" (267mm) | 4.02" (102mm) | 1,435 fps (437 mps) | 25.3 |
| 30,000 yards (27,432 m) | 9.08" (231mm) | 5.11" (130mm) | 1,400 fps (427 mps) | 32.8 |
| 35,000 yards (32,004 m) | 7.35" (187mm) | 7.18" (182mm) | 1,427 fps (437 mps) | 44.5 |
| Mount / Turret Data | |
| Designation | 3-gun Turret |
| 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 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 was to increase the rate of fire and to provide 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 testily 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 resold, as former crewmembers have informed me that there were no major problems during their war-cruises. | |
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