During target practice in 1916, USS Michigan (B-27) cracked the chase hoops on two of her guns. An investigation of similar guns on USS South Carolina (B-26) showed that copper deposits from the projectile driving bands had narrowed down the bores, which slowed the projectiles and thus greatly increased the barrel pressures (problem known as "copper choke"). Lapping heads for guns 12" (30.5 cm) and larger were issued to all ships for removing these deposits. Later, these were replaced by wire and pisaba brushes.
Under the provisions of the Washington Naval Limitation Treaty of 1922, most of the ships armed with these guns were scrapped during the mid-1920s. Many of their guns were then transferred to the US Army where they were employed as coastal artillery. At the end of World War II, some of these guns were sold to Brazil for use in their coastal batteries.
In 1908 AP projectiles were fitted with a longer ballistic cap of 7crh which improved their penetration ability at longer ranges.
Constructionally, the Mark 5 was essentially a lengthened 12"/40 (30.5 cm) Mark 4. The Mark 6 was very similar except that it had seven hoops vs. six in the Mark 5. It is not clear if any Mark 6 guns were actually used in service.
A note on sources: Appendix D of "U.S. Battleships: An Illustrated Design History" by Norman Friedman states that the USS Connecticut (B-18) and USS Mississippi (B-23) classes carried 12"/40 (30.5 cm) guns. Other sources disagree and state that they carried 12"/45 (30.5 cm) guns. Through an analysis of several photographs and, ironically, aided by notes in another book by Dr. Friedman, "US Naval Weapons," I have concluded that these ships actually carried the 12"/45 (30.5 cm) gun and that Dr. Friedman in "U.S. Battleships" is in error.
The data that follows is specifically for the 12"/45 (30.5 cm) Mark 5 Mod 9.
Aft turrets of USS Delaware B-28 about
|Designation||12"/45 (30.5 cm) Mark 5 and Mark 6|
|Ship Class Used On||Connecticut (B-18), Mississippi (B-23), South Carolina (B-26), Delaware (B-28) and Florida (B-30) Classes|
|Date Of Design||about 1903|
|Date In Service||1906|
|Gun Weight||53 tons (54 mt)|
|Gun Length oa||N/A|
|Bore Length||540 in (13.716 m)|
|(72) 0.075 in deep x 0.2736 in (1.91 mm x 6.95 mm)|
|Lands||0.25 in (6.35 mm)|
|Twist||Most Mark 5 mods: Increasing RH
0 to 1 in 25
Mark 5 Mod 10: Uniform RH 1 in 32
Mark 6: Increasing RH 0 to 1 in 25
|Rate Of Fire||2 - 3 rounds per minute|
|Note: Early mods had "hook" rifling, where the grooves were not uniformly shaped. The values given above for grooves and lands are for the later "rib" rifling.|
|Projectile Types and Weights||AP - 870 lbs. (394.6 kg)
Common - 870 lbs. (394.6 kg)
|Bursting Charge||AP - 24.0 to 24.7 lbs. (10.9 to 11.2 kg)
Common - N/A
|Propellant Charge||310 lbs. (140.6 kg) SPD|
|Muzzle Velocity||2,700 fps (823 mps)|
|Working Pressure||17 tons/in2 (2,681 kg/cm2)|
|Approximate Barrel Life||175 rounds|
|Ammunition stowage per gun||Connecticut class: 60 rounds
Mississippi class: 71 rounds
Others: 100 rounds
|Note: Common was obsolete by 1915 and no longer in production.|
|Elevation||With 870 lbs. (394.63 kg) AP Shell|
|Range @ 15 degrees||about 20,000 yards (18,290 m)|
|Range @ 47 degrees
(as coastal artillery)
|30,000 yards (27,432 m)|
|6,000 yards (5,490 m)||
|9,000 yards (8,230 m)||
|12,000 yards (10,920 m)||
|This data is for face-hardened Harvey plates from "Ordnance Data Sheets" of 1905 and is for the older shell design.|
|6,000 yards (5,490 m)||
|9,000 yards (8,230 m)||
|12,000 yards (10,920 m)||
|This data is from "Elements of US Naval Guns" of 1918 and is for the 7crh projectile. Data is corrected for angle of fall and may also refer to harder armor than that used for the 1905 data.|
Connecticut (2) : Mark 6
Mississippi (2), South Carolina (4) and Delaware (5): Mark 7
Florida (5): Mark 8
427 tons (434 mt)
|Elevation||-5 / +15 degrees|
|Rate of Train||N/A|
|Loading Angle||Mark 6: 0 degrees
Mark 7 and Mark 8: Any angle
1) As in all US pre-dreadnought designs, these turrets had endless chain hoists running on unenclosed rails direct from the lower chambers to the loading position. After an accident on USS Missouri (B-11) in April 1904 that saw a flare-back from a gun breech go straight to the handling room and which killed five officers and twenty nine men, automatic shutters were installed to separate the ends of the hoists. In April 1906, exposed switch gear on USS Kentucky (B-5) caused a powder burn that killed 10 officers and men. As a result, all electrical equipment that might cause a spark hazard was removed from all USN mountings between 1907 and 1908. Bulkheads were installed in the turrets between guns and breech gas ejectors were fitted to prevent flarebacks in the future. These changes became standard on all subsequent designs.
2) The Mark 7 introduced all-electric power to USN turrets, although electric motors had been widely used in previous designs.
3) The Mark 8 was similar to the Mark 7 and was the last US large-caliber turret design to have a mark number.
4) The thickness of the armor protection was the main difference in turret weights between the different mark numbers.
5) USS South Carolina (B-26) was the first US battleship to have two-stage projectile and powder hoists. Cars now carried both shell and powder from lower to upper handling room, where they were rammed by hand through flap-covered openings into upper hoist cars, which in turn carried them to the guns. Training was by two 25 hp motors and each gun had a 15 hp motor for elevation. A 10 hp motor drove a chain rammer carried on an extension of the cradle. All electric motors were duplicated by manual hand drives as a backup. In the upper handling room, ammunition was transferred between hoists by a rammer powered by a 10 hp electric motor.
6) The hoists for the South Carolina class were developed by the Washington Navy Yard and proved to be satisfactory. However, the ones for the Delaware class (B-28) were designed by Bethlehem Steel and proved so troublesome that they were removed. Improvised fittings in their place consisted of rope whips.
7) These turrets used the "grass-hopper" counter recoil system whereby a spring box, located under the gun pit, was connected via two heavy, pivoted arms to the gun yoke. See 10"/40 (25.4 cm) datapage for a sketch.
8) USS South Carolina (B-26) was the first US ship to use the "hyposcope," a new type of gunsight which used a series of mirrors to remote the point of aim. This device was rigidly connected to the gun trunnions and set into the side of the turret. Previous gunsight designs were located on the turret roof in hoods and were connected by a complicated linkage to the guns. This resulted in "lost motion" as the slack in the linkage connections meant that the gunsights could not exactly follow the gun elevations. At short ranges, this lost motion was not significant, but as the range increased, the accuracy became worse. With the hyposcope, a range transmitted from any gun to the other guns on the ship meant that they could all now be pointed at the correct elevation, hence the patterns became tighter. Each gun in the turrets elevated separately, but there was a single sightsetter in each turret who was responsible for setting the correct range for both hydroscopes. A third sight was provided for the gun mount trainer and this projected up through the turret roof. It has been suggested that these characteristic turret-side sights on US warships were adopted in order to allow superfiring guns to shoot directly over the lower turrets, but this does not seem to be the case. The lead designer, Chief Constructionist Washington Capps "appears neither to have feared trouble from the blast nor to have considered end-on fire important" - Norman Friedman in "U.S. Battleships." Experiments to verify the new superfiring battleship design were conducted using the monitor USS Florida (M-9) modified with one of her 12" (30.5 cm) guns re-located in the superstructure so that it fired over her turret. The turret itself was modified to represent those planned for USS South Carolina with the exception that the gunsights were mounted further aft so as to be closer to the muzzle of the superfiring gun. Tests conducted on 6 - 15 March 1907 with full charges and with the sights directly below the superfiring gun were completely successful with the official report stating that "the shock felt was trifling, and the officer at the sight felt no jar, nor was his vision of the horizon interrupted otherwise than by the smoke of the discharge." The only design change made to the turret designs on subsequent ships as a result of these tests was a slight thickening of the turret roofs.
18 September 2008 - Benchmark
10 July 2014 - Added Mount / Turret comments about hyposcope gunsights on USS South Carolina and tests with the monitor USS Florida
08 May 2015 - Redid photograph of USS Delaware, other minor changes