12in Mortar on Railway Mount Model 1918 (10 caliber)

The 12in Mortar on Railway Mount Model 1918 carried a 12in mortar on a fully traversing barbette. Several were almost completed by the end of the First World War, and it remained in use between the wars, before the carriage was used to carry surplus 8in naval guns.

This railway gun was armed with a 12in coast defence mortar Model 1890 Mark I. This was 10 calibres long, with an interrupted threat breech block and mechanical firing mechanism. It used a hydro-pneumatic recoil system, with two hydraulic recoil cylinders on the bottom of the cradle and one pneumatic recuperator cylinder above it. The recuperator cylinder was fixed to the cradle, with a hollow piston that was attached to the gun and a floating piston that passed through the hollow cylinder. Both pistons were forced back when the gun fired. Air pressure behind them then pushed the floating piston back, forcing the hollow piston and the gun back into their original position.

Elevation was provided by a segmental circular rack attached to the bottom of the cradle, connected by gears to a hand wheel. One turn of the hand wheel moved the mortar 1.004 degrees up or down. The gun could be fired at any angle of elevation between 20 and 65 degrees, but was most efficient above 35 degrees and dangerously unstable below 25 degrees.

The gun and carriage were carried between steel side frames that were supported by a racer casting, which was supported by conical traversing rollers that sat on a circular rack on the base plate. A pinion on the base of the racer casting meshed with the rack, and connected by gears to an operating wheel. One turn of the hand wheel rotated the mortar 0.837 degrees.

The railway car was a low loader type, similar to the one used on the 8in Gun on railway car model 1918. However this wasn’t quite strong enough to cope with the 12in mortar, and four fifths of the cars were built to a stronger design. The heavier weapon also meant that heavier bogies, with six wheels, were used at each end of the carriage.

The use of the same carriage meant that the 12in mortar also used the same anchorage system as the 8in gun. The entire gun carriage had to be jacked up. Two lines of ‘H’ beams were placed lengthways under the carriage, and six wooden cross beams were placed across them. The carriage was then lowered onto the cross beams. Outriggers were provided to keep the carriage stable.

None of the 12in mortar railway carriages had been completed by the end of the First World War. A number of them were completed in the immediate post-war period.

The 12in mortar remained in service with the coastal artillery for some time after the First World War. Some were used by the 41st Artillery (Railway) on Hawaii in the 1920s. It was then decided to use the 12in mortar mounts to carry surplus 8in naval guns, but work on this project was delayed until 1941-42. The resulting 8in Mark VI Mod 3A2 on Railway Mount M1A1 was used on coastal defence duties in the Continental United States


12in Mortar on Railway Carriage Model 1918



Barrel Length


Gun Length

3.58m (141.12in)

Life of barrel at full charge

1,150 rounds

Weight for transport


Weight in action

80,340kg (177,000lb)


20 to 65 degrees


360 degrees

Shell Weight

474kg (1,046lb) Short cap naval shell, 26.5kg explosives
374.01 (824lb) short cap naval shell, 14.84kg explosives
317kg (700lb) HE shell, 42.8kg explosives
317.73kg (700lb) long point naval shell, 10.98kg explosives

Muzzle Velocity

457 m/s (1,500ft/ sec)

Maximum Range

13.98km (15,300 yards)

Rate of Fire

25 rounds/ hour


Books on the First World War | Subject Index: First World War

How to cite this article: Rickard, J (16 January 2019), 12in Mortar on Railway Mount Model 1918 (10 caliber) , http://www.historyofwar.org/articles/weapons_12in_mortar_railway_mount_1918.html

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