RML 12 inch 25 ton gun

Ordnance RML 12 inch 25 ton gun

A 12-inch (305 mm) 25-ton gun in an armoured turret aboard the ram HMS Hotspur. A shell is suspended near the muzzle ready for loading.
Type Naval gun
Coast defence gun
Service history
Used by Royal Navy
Production history
Designed 1864 - 1866
Manufacturer Royal Arsenal
Unit cost £1,716[1]
Produced 1866 - 187?
Variants Mk I, Mk II
Weight Mk I : 23.5 long tons (23,900 kg)
Mk II : 25 long tons (25,000 kg)
Barrel length 145 inches (3.7 m) (bore + chamber)[2]

Shell 600 to 608.4 pounds (272.2 to 276.0 kg) (Palliser)
497 pounds (225.4 kg) (Common & Shrapnel)
Calibre 12-inch (304.8 mm)
Muzzle velocity 1,300 feet per second (400 m/s)[3]

The RML 12 inch 25 ton guns were large rifled muzzle-loading guns of mid-late 1800s used as primary armament on British ironclad turret battleships and coastal monitors, and also ashore for coast defence. They were the shorter and less powerful of the two 12-inch (305-mm) British RML guns, the other being the 35-ton gun.


Mk I & II gun construction
Rear view of a 12-inch (305 mm) 25-ton gun aboard the ram HMS Hotspur.

Mark I

Four guns were first made in 1866 with a toughened mild steel tube surrounded by multiple wrought iron coils on the original Armstrong pattern.

Mark II

While strong, the multiple coils were considered too expensive for construction in quantity. From 1867 guns were built on the simplified and hence cheaper "Fraser" system involving fewer but larger coils similar to the 10-inch (254-mm) Mk II gun. The guns were not considered a success, with the rifling twist of 1 in 100 increasing to 1 in 50 considered insufficient for accuracy, and guns were retubed in 11-inch (279-mm) calibre when their bores wore out.[4]

Guns were mounted on :


When the gun was first introduced projectiles had several rows of "studs" which engaged with the gun's rifling to impart spin. Sometime after 1878, "attached gas-checks" were fitted to the bases of the studded shells, reducing wear on the guns and improving their range and accuracy. Subsequently "automatic gas-checks" were developed which could rotate shells, allowing the deployment of a new range of studless ammunition. Thus, any particular gun potentially operated with a mix of studded and studless ammunition.

The gun's primary projectile was 600 to 608-pound (272- to 275-kilogram) "Palliser" armour-piercing shot, fired with a "Battering charge" of 85 pounds (38.5 kilograms) of "P" (gunpowder) or 67 pounds (30.4 kilograms) "R.L.G." (gunpowder) for maximum velocity and hence penetrating power. Shrapnel and Common (exploding) shells weighed 497 pounds (225.5 kilograms) and were fired with a "Full charge" of 55 pounds (25 kilograms) "P" or 50 pounds (22.7 kilograms) "R.L.G.".[5]

See also

Surviving examples

Notes and references

  1. Unit cost of £1,715 13 shillings 5 pence is quoted in "The British Navy" Volume II, 1882, by Sir Thomas Brassey. Page 38
  2. Treatise on Construction of Service Ordnance 1877, page 292
  3. MV of 1,300 feet/second firing 600-pound projectile with "Battering charge" of 85 pounds "P" (gunpowder) is quoted in "Treatise on Construction of Service Ordnance 1877", page 348. MV of 1,292 feet/second firing 608 lb 6 oz projectile with "Battering charge" of 85 pounds "P2" (gunpowder) is quoted in "Text Book of Gunnery 1887" Table XVI
  4. Treatise on Construction of Service Ordnance 1877, pages 92-94
  5. Treatise on Ammunition 1877, pages 191,194, 220


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