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.

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

While strong, the multiple coils were considered too expensive for construction in quantity.

Some time 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.