United States of America
8"/45 (20.3 cm) Mark 6
Updated 13 December 2008

Part of the generation of smokeless-powder guns developed at the end of the Spanish American War and much stronger than the previous 8"/40 (20.3 cm) Mark 5.  Used as secondaries on the last US pre-dreadnoughts and as main guns on armored cruisers.  A number of older armored cruisers were rearmed with this gun during refits in the early 1900s.

This weapon was unusual in that some pre-dreadnoughts used them in two-level turrets, with the 8" (20.3 cm) guns on top and a larger caliber below.  Although ultimately unsuccessful in this configuration, the experience gained led to the very successful development of superfiring main-caliber turrets on USS South Carolina (B-26), the first "dreadnought" with this mounting scheme.

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

The scrapping of pre-dreadnoughts as required by the Washington Naval Limitation Treaty made many of these guns surplus.  About two dozen were reused as coastal artillery on Army mountings and given new, lighter and more streamlined projectiles.

Constructed of A tube, jacket, four hoops, a locking ring and a liner, all of nickel steel with a Welin breech block.  A total of eight Mods, 6/0 to 6/7, were used with the differences being in the liners, breech mechanisms, chambers and rifling.


USS San Diego ACR-6 (ex-USS California) about 1917
Officers are Adm. W.B. Caperton and his staff
U.S. Naval Historical Center Photograph # NH 83793

Click here for additional pictures
Gun Characteristics
Designation 8"/45 (20.3 cm) Mark 6
Ship Class Used On As built:  Virginia (B-13), Connecticut (B-18) and Mississippi (B-23) classes

As rearmed:  USS New York (ACR-2) and Pennsylvania (ACR-4) classes

Date Of Design about 1900
Date In Service 1906
Gun Weight 18.8 tons (19.2 mt)
Gun Length 368.0 in (9.373 m)
Bore Length 360 in (9.144 m)
Rifling Length 288.79 in (7.335 m)
Grooves 0.07 in (1.78 mm) deep
Lands N/A
Twist Mod 0:  Uniform RH 1 in 25
Others:  Increasing RH 1 in 44.4 to 1 in 25 at the muzzle
Chamber Volume N/A
Rate Of Fire 1 - 2 rounds per minute
Note:  Firing was by both percussion and electrical means.
Type Bag
Projectile Types and Weights Naval AP - 260 lbs. (118 kg)
Naval Common - 260 lbs. (118 kg)

Army AP Mark 20 - 260 lbs. (118 kg)
Army HE M103 - 240 lbs. (109 kg)

Bursting Charge Naval AP - 6.0 to 6.2 lbs. (2.7 to 2.8 kg) Explosive D
Naval Common - about 15.6 lbs. (7.1 kg)

Army AP - about 6.5 lbs. (2.9 kg)
Army HE - N/A

Projectile Length N/A
Propellant Charge Navy - 98.5 lbs. (44.7 kg) SPD
Army - N/A
Muzzle Velocity Naval AP - 2,750 fps (838 mps)
Naval Common - 2,750 fps (838 mps)

Army AP (normal charge) - 2,100 fps (640 mps)
Army AP (super charge) - 2,750 fps (838 mps)

Army HE (normal charge) - 2,150 fps (655 mps)
Army HE (super charge) - 2,840 fps (866 mps)

Working Pressure 17.0 tons/in2 (2,680 kg/cm2)
Approximate Barrel Life N/A
Ammunition Stowage per gun Virginia:  125 rounds
Connecticut:  100 rounds
Mississippi:  111 rounds
Pennsylvania:  125 rounds
Note:  During World War I, a "flat-nose" shell was developed for use against submarines.  The flat nose allowed the projectile to travel through water with reasonable accuracy.  Assuming an overall shell weight of 260 lbs. (118 kg), this shell had a burster of about 68 lbs. (31 kg).
With Naval 260 lbs. (118 kg) AP
MV = 2,750 fps (838 mps)
Striking Velocity
Angle of Fall
3.3 degrees
7,000 yards (6,400 m)
1,876 fps (572 mps)
4.0 degrees
8,000 yards (7,320 m)
1,773 fps (540 mps)
4.3 degrees
8,500 yards (7,720 m)
1,724 fps (525 mps)
20.1 degrees
22,500 yards (20,575 m)
42 degrees
(Army RR Gun)
24,900 yards (22,770 m)
With Army 240 lbs. (109 kg) AP
45 degrees
(Army RR Gun)
35,300 yards (32,280 m)

1) Naval mountings had a maximum elevation of +20 degrees.

2) Time of flight for Navy 260 lbs. (118 kg) AP with MV = 2,750 fps (838 mps)
   7,000 yards (6,400 m):  9.3 seconds
   8,000 yards (7,320 m):  11.0 seconds
   8,500 yards (7,720 m):  11.8 seconds

Armor Penetration With 260 lbs. (118 kg) AP Shell
Side Armor
Deck Armor
0 yards (0 m)
12.0" (305 mm)
6,000 yards (5,490 m)
7.58" (193 mm)
9,000 yards (8,230 m)
5.48" (139 mm)
12,000 yards (10,920 m)
4.63" (118 mm)
The above data is from "Ordnance Data Sheets" of 1905 for face-hardened Harvey plates from and is for the older shell design.  "U.S. Armored Cruisers:  A Design and Operational History" reports that this same shell could penetrate 4.4" (112 mm) of Krupp nickel-chromium armor at 9,000 yards (8,230 m).
Side Armor
Deck Armor
6,000 yards (5,490 m)
8.6" (218 mm)
9,000 yards (8,230 m)
6.6" (168 mm)
12,000 yards (10,920 m)
5.0" (127 mm)
The above 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 used for the 1905 data.
Mount / Turret Data
Designation Two-gun Turrets
   Virginia (2), Connecticut (4) and Mississippi (4) :  Mark 12
   New York (2) and Pennsylvania (2) as refitted:  Mark 12

Dual-Caliber Turrets
   Virginia (2):  Mark 5 (details under the 12" (30.5 cm) mountings)

Army Railway Mount:  M1A1

Weight  Mark 12
   Pennsylvania:  147 tons (149 mt)
   Virginia:  151 tons (153 mt)
   Connecticut:  149 tons (151 mt)
Elevation Mark 12:  -7 / +20 degrees

Mark 5:  -7 / +20 degrees

M1A1 (firing):  0 / +45 degrees

Elevation Rate N/A
Train Naval Mounts:  +135 / -135 degrees

Army M1A1:  360 degrees continuous

Train Rate 6 degrees per second
Gun recoil 28.5 in (72.4 cm) max
Loading angle Mark 12:  0 degrees

M1A1:  - 5 degrees


1) These turrets were balanced, which means that the center of rotation was also the center of weight.  The Mark 12 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.

2) The Mark 12 was the first USN turret with an inclined glacis plate, an invention by Chief Constructor Philip Hichborn.  The cylindrical turret used on previous ships had vertical sides, thus requiring large gunports in order to achieve even a limited elevation.  The inclined face of the Mark 12 meant that high elevations could be achieved with gun ports only slightly larger than the outside gun diameter, resulting in greatly improved protection.  This basic design was so successful that the USN continued to use it for gun mounts and turrets until after World War II.

3) Training, elevation, hoists and rammers were all electrically powered.  It took 15 minutes to train the Mark 12 turrets through 270 degrees by hand while it took only 45 seconds to do the same job with electric motors.

4) During modifications in 1905, New York had all six of her 8"/35 (20.3 cm) guns removed and was refitted with four 8"/45 Mark 6 guns in two Mark 12 turrets.  The Pennsylvania class traded their four 8"/35 (20.3 cm) guns for four 8"/45 (20.3 cm) Mark 6 guns.

5) Between 1907 and 1909 these turrets were extensively modified.  Almost all electrical gear that could create sparks was moved from the turrets and ammunition spaces and placed into compartments below the armored deck.  Electrical equipment that was absolutely required to remain was made flameproof.  Automatic shutters were fitted in the ammunition supply tubes between the turret and magazines in order to increase flash protection.  Longitudinal bulkheads were fitted to separate the guns into individual compartments.  Electric powered ammunition hoists and rammers were removed and replaced with handworked gear to increase the rate of fire at a cost of increased manning.

Data from
"Naval Weapons of World War Two" by John Campbell
"US Naval Weapons," "U.S. Cruisers:  An Illustrated Design History" and "U.S. Battleships:  An Illustrated Design History" all by Norman Friedman
Seacoast Fortifications of the United States:  An Introductory History" by Emanuel Raymond Lewis
"U.S. Armored Cruisers:  A Design and Operational History" by Ivan Musicant
"The Engineer" 27 April 1917 (as quoted in Warship International No. 1, 1994)
"Naval Ordnance - A Text Book" revised in 1915 by Lt. Cmdr. Roland I. Curtain and Lt. Cmdr. Thomas L. Johnson
"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
FM 4-49, 8-Inch Gun, Mark VI, Modification 3A2, on Railway Mount M1A1 August 6 1942