RML 12.5 inch 38 ton gun

The RML 12.5 inch guns were large rifled muzzle-loading guns designed for British battleships and were also employed for coast defence.

Design
The gun originated from a desire for a longer 12-inch gun than the existing RML 12 inch 35 ton gun. Experiments in 1874 with both 12-inch and 12.5-inch versions 3 feet longer than the existing 12-inch gun showed the 12.5 inch calibre was more suitable, and further experiments showed a projectile of 800 pounds could be fired with a charge of 130 pounds of P2 gunpowder without undue strain. The same construction as in the existing 12-inch 35-ton gun was used : a mild steel "A" tube toughened in oil, surrounded by wrought iron "B" tube, triple coil in front of the trunnion, coiled breech-piece and breech coil. This was approved in January 1875.

The gun was rifled on the "Woolwich" pattern of a small number of broad shallow rounded grooves, with 9 grooves increasing from 0 to 1 turn in 35 calibres ( i.e. 1 turn in 437.5 inches).

Mark II had an enlarged powder chamber and attained higher muzzle velocity and slightly longer range.

This gun was the final development of large British rifled muzzle-loading guns before it switched to breechloaders beginning in 1880. It was succeeded in its class on new battleships by the BL 12-inch Mk II gun. 

Naval service
Guns were mounted on HMS Dreadnought commissioned in 1879, HMS Agamemnon commissioned in 1883, and HMS Ajax commissioned in 1885, the last British warships completed with muzzle-loading guns.

Surviving examples

 * At Hurst Castle, UK
 * At Fort Nelson, Portsmouth, UK
 * Outside Fort Albert, Isle of Wight
 * No 22 of 1876 outside Calbourne Mill, Isle of Wight, originally at Cliff End Battery
 * An unpreserved gun at Fort Delimara, Malta