The very fast burning, smoky propellants of the 1800s gave way to slower burning "smokeless" powders near the turn of the century. These propellants gasified almost completely and could accelerate a projectile all the way along a gun of 40 or more calibers in length. Thus, the high-velocity naval gun was born. A side benefit was the flatter trajectory, which led to more accurate firing at longer ranges, which in turn led to centralized fire control and then ultimately to the "Dreadnought" designs.

In the case of the USN, the development of smokeless powder led to a new generation of long-barrel naval guns, including the 12"/40 (30.5 cm). This gun was used on several pre-dreadnoughts and monitors. The early guns suffered from excessive bore erosion, necessitating a reduction in the propellant charge to lower the muzzle velocity.

Perhaps the most interesting use of this weapon was on the two-level, dual-caliber turrets that were unique to the US Navy. The concept behind these mountings was that the large caliber weapons fired slowly, perhaps once every ninety seconds, while the 8"/45 (20.3 cm) guns fired about three times as fast. That meant that the smaller guns could fire one or two shots at a second target while the larger guns were reloading. Unfortunately for this design, better crew training and more careful attention to details led to a rapid improvement in the rate of fire for large caliber weapons, eliminating the advantage of the smaller guns over the larger ones. As a result, the dual-caliber turrets could really only fire at a single target and the smoke from both calibers interferred with sighting unless they were both fired together. The concept did lead to superfiring main-caliber turrets when the South Carolina class dreadnoughts were designed a few years later.

In 1908 AP projectiles were fitted with a longer ballistic cap of 7crh which improved their penetration ability at longer ranges.

Mark 3 consisted of A tube, jacket and eight hoops. The Mark 4 was very similar with a smaller chamber for the reduced propellant charge.

Gun Characteristics

Designation 12"/40 (30.5 cm) Mark 3 and Mark 4
Ship Class Used On Arkansas (M-7), Maine (B-10) and Virginia (B-13) classes
Date Of Design 1899
Date In Service 1902
Gun Weight 116,480 lbs. (52,834 kg) (including breech)
114,960 lbs. (52,145 kg) (without breech)
Gun Length oa N/A
Bore Length 480 in (12.192 m)
Rifling Length N/A
Grooves N/A
Lands N/A
Twist Increasing RH 0 to 1 in 25
Chamber Volume N/A
Rate Of Fire 1 As commissioned: 0.66 rounds per minute
After about 1906: 2 rounds per minute
  • ^The original Rate of Fire was greatly improved by more careful loading practices and better training.


Type Bag
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) Explosive D
Common - N/A
Projectile Length N/A
Propellant Charge 237.5 lbs. (107.7 kg) [for 2,400 fps (732 mps) MV]
Muzzle Velocity 1a Original design: 2,800 fps (853 mps)
First Derating: 2,600 fps (640 mps)
Final Derating: 2,400 fps (732 mps)
Working Pressure 16.0 tons/in2 (2,520 kg/cm2)
Approximate Barrel Life 100 - 150 rounds at 2,400 fps (732 mps)
Ammunition stowage per gun 60 rounds
  • ^Muzzle velocity was reduced in order to improve life and reduce dispersion.


Range of projectiles fired at new gun muzzle velocities
Elevation With AP Shells
15.5 degrees
(maximum elevation of turrets)
19,000 yards (17,370 m) 1b
  • ^It should be mentioned that this maximum range was of little use at the time these ships were built. Fire control systems and rangefinders capable of accurately firing at ranges over 10,000 yards (9,140 m) were nonexistent.

Armor Penetration with "old-style" AP Shell

Range Side Armor Deck Armor
6,000 yards (5,490 m) 14.6" (371 mm) ---
9,000 yards (8,230 m) 11.6" (295 mm) ---
12,000 yards (10,920 m) 9.4" (239 mm) ---
  • This data is for face-hardened Harvey plates from "Ordnance Data Sheets" of 1905 as published in "US Naval Weapons" and is for the older shell design. These figures are for a muzzle velocity of 2,600 fps (640 mps).

Armor Penetration with "new-style" AP Shell

Range Side Armor Deck Armor
6,000 yards (5,490 m) 12.9" (328 mm) ---
9,000 yards (8,230 m) 10.7" (272 mm) ---
12,000 yards (10,920 m) 8.7" (221 mm) ---
  • This data is from "Elements of US Naval Guns" of 1918 as published in "US Naval Weapons" and is for the 7crh projectile at a muzzle velocity of 2,400 fps (732 mps). Data is corrected for angle of fall and may also refer to harder armor than that used for the 1905 data.

Mount/Turret Data

Designation Two-gun Turrets
   Arkansas (2) and Maine (2): Mark 4 1e

Dual-Caliber Turrets
   Virginia (2): Mark 5 2e

Weight Mark 4: N/A
Mark 5: 604 tons (614 mt)
Elevation Mark 4: -3 / +15 degrees
Mark 5: -7 / +20 degrees
Rate of Elevation N/A
Train about -150 / +150 degrees
Rate of Train N/A
Gun Recoil N/A
Loading Angle 0 degrees
  • ^The Mark 4 introduced the barbette/gunhouse style of construction to US ships and used a spring return system.
  • ^The Mark 5 was the 12"/8" (30.5 cm/20.3 cm) dual-caliber turret.
  • 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 were 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. These changes became standard on subsequent designs.
  • 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.

Additional Pictures


"US Battleships: An Illustrated Design History" and "US Naval Weapons" both by Norman Friedman
"Naval Ordnance - A Text Book" revised in 1915 by Lt. Cmdr. Roland I. Curtain and Lt. Cmdr. Thomas L. Johnson

Page History

15 September 2008 - Benchmark
21 April 2015 - Fixed link, added some new links to other Weapon datapages
09 May 2015 - Redid photograph of USS Ohio
19 July 2016 - Converted to HTML 5 format
20 December 2018 - Reorganized notes