12"/25 (30.5 cm) Marks I and II
Updated 15 October 2012

Early breech loaders using black powder for propellant.  The Mark I was considered to be the first acceptable large-caliber BL design produced by Woolwich.  HMS Colossus was originally to be armed with muzzle-loading 12 inch (30.5 cm) guns but she instead became the first British battleship to have BL main guns, steel construction and steel armor all in one ship.

The design of these guns was faulty and one burst on trial in HMS Collingwood on 4 May 1886, even though she was firing 3/4 charges.  The failure caused the "A" tube to break into pieces in front of the "B" coils.  As a result, HMS Colossus was directed not to fire her main guns during her maiden cruise and all of these guns were withdrawn from service to be casehooped.  They were then designated as Mark I* and mounted in the "No Man's Land" and "Horse Sand" coastal defense forts at Spithead.  The ships were rearmed with newer, stronger 12"/25 (30.5 cm) guns.

Constructed of steel "A" tube into which the breech-block screwed, a wrought iron breech piece, three "B" coils of which the first was medium carbon steel and the others made of mild steel, one wrought iron "C" coil and a wrought iron jacket.  There were no coils over the last 8 feet (2.44 m) of "A" tube at the muzzle end.  All of the wrought iron parts were machined from hammer-welded coils.  Mark I was a trunnioned gun for use in coastal defense batteries while the Mark II had thrust collars for naval saddle mountings, otherwise the two marks were identical.  A total of 5 Mark I and 11 Mark II guns were manufactured.

Actual bore length was 25.13 calibers.


HMS Collingwood in 1882 with 12"/35 (30.5 cm) Mark II guns.  She was later rearmed with 12"/25 (30.5 cm) Mark Vw guns.
Note the open barbettes

Gun Characteristics
Designation 12"/25 (30.5 cm) Marks I and II
Ship Class Used On Mark II: Colossus, Conqueror and Collingwood
Date Of Design 1882
Date In Service 1885
Gun Weight 43.8 tons (44.5 mt) inc. breech
Gun Length oa 328.5 in (8.344 m)
Bore Length 301.6 in (7.660 m)
Rifling Length N/A
Grooves N/A
Lands N/A
Twist N/A
Chamber Volume N/A
Rate Of Fire 0.3 - 0.5 rounds per minute
Note:  At target practice in 1887 HMS Colossus made record shooting by getting off four shots from one gun in six minutes and scoring three hits at 1,500 yards (1,370 m) while traveling at full speed (about 16 knots).
Type Bag
Projectile Types and Weights AP - 714 lbs. (324 kg)
Bursting Charge N/A
Projectile Length N/A
Propellant Charge
(see Note)
295 lbs. (134 kg) P.Br. (Prismatic Brown - cocoa)
Muzzle Velocity 1,914 fps (583 mps)
Working Pressure N/A
Approximate Barrel Life N/A
Ammunition stowage per gun N/A
Note:  The original charge was 400 lbs. (181.4 kg) black powder and then reduced to 290 lbs. (131.5 kg) and finally to 220 lbs. (99.8 kg), the final figure being the charge used when the gun exploded.  After rehooping, the charge was changed to the figure and type above.
Elevation With 714 lbs. (324 kg) Projectile
Range @ 12.5 degrees 9,400 yards (8,600 m)
Armor Penetration with 714 lbs. (324 kg) Projectile
Range Vertical Iron Plate
1,000 yards (9,140 m) 20.5 in (52 cm)
Note:  Data from "British Battleships:  1850 - 1950."
Mount / Turret Data
Designation Twin Turret
   Colossus (2), Conqueror (1)

Twin Barbette
   Collingwood (2)

Weight N/A
Elevation about - 3 / +10 degrees
Rate of Elevation N/A
(see Note 2)
Colossus:  0 / about 170 degrees
Conqueror:  -45 / +45 degrees
Collingwood:  -90 / +90 degrees
Rate of Train N/A
Gun Recoil N/A
Loading Angle 13 degrees

1) The guns on these ships could only be loaded when the turret was trained to 0 degrees.  The breech end of the weapons were lowered below the deck level by a hydraulic ram and then the charges were rammed into the breech from a compartment outside of the rotating structure.

2) The Colossus class had their guns in an amidships citadel with the turrets staggered.  This allowed them to fire across the beam although it is not clear if blast effects limited their use in such a manner.  They could also fire directly ahead but not directly astern.  For the Conqueror class, owing to blast effects on the bridge, it was found to be inadvisable to fire these guns abaft the beam, so the arcs of fire were limited to the figures above.  It is noted that Collingwood had no problems from blast effects when firing abaft the beam.

3) The designers worried that the enclosed nature of these turrets would trap propellant smoke, but it was found in practice that the long interval between when the guns were fired and they returned to the loading position gave sufficient time for the fumes to dissipate out the muzzles before the breeches were opened.

Data from
"Warrior to Dreadnought:  Warship Development 1860-1905" by D.K. Brown
"British Naval Guns 1880-1945 No 4" article by John Campbell in "Warship Volume V"
"The Big Gun:  Battleship Main Armament 1860-1945" by Peter Hodges
"British Battleships:  1860 - 1950" by Oscar Parkes
Page History

19 October 2007 - Benchmark
30 January 2009 - Fixed minor typographical errors
29 August 2010 - Added details about ship design and details of charge weight
15 October 2012 - Corrected typographical error