Obusier de 370 modèle 1915

With the success of the Obusier de 400 Modèle 1915/1916 in mind, the French Army requested more of these guns for service. The mle 1915 was a conversion of an existing naval gun the 340 mm (13 in) L/45 mle 1887 by shortening them to 25 calibers and boring them out to 400 mm, but these gun barrels were in short supply. An alternative was to perform the same kind of conversion to Canon de 305 mm Modèle 1887 gun barrels to create 370 mm (15 in) L/25 guns. It was proposed to convert eight gun barrels to railway artillery with another four spare barrels. The barrels for these guns would come from decommissioned ships of the Charles Martel class of pre-dreadnought battleships and Bouvines class coastal defense ships. The work of building the guns was split between Schneider and Batignolles each producing four guns.[2]

Like the earlier Obusier de 400 mm mle 1915/1916 the Obusier de 370 mm mle 1915 was considered a howitzer instead of a gun, so range and velocity were not primary considerations. Instead, high angle fire, projectile weight, and explosive yield were the primary considerations. The gun had a Welin breech block and the gun used separate loading bagged charges and projectiles. There were slight differences between the two groups of four guns. For example, a 370 mle 1915 with gun barrels from the battleship Charles Martel weighed 127 tonnes, while the others from the Bouvines class of coastal defense ships weighed 134 tonnes. Whether there were any ballistic differences between the finished pieces is unknown.[2]

Compare the original naval gun and the modified gun, the mle 1915 barrel is 33% shorter, the projectile weighs 2-3 times as much, the muzzle velocity is only 60% of the original, but the range of the projectile is actually greater due to firing at greater angles of elevation.

A diagram showing the Batignolles carriage system.

Each gun sat on a rectangular steel base which was suspended from two railroad bogies with four axles each. The number of axles was determined by the weight limit for European railways of 17 tonnes per axle and 17x8 = 136 tonnes.[1] The howitzers had a hydro-pneumatic cradle recoil system where the cradle recoiled up a slightly inclined rear deck which helped return the howitzer to battery after firing. The carriage used a car traversing mount so traverse was limited to a few degrees each side of the howitzers centerline. To load the gun the barrel was lowered to -6° and a loading ramp to the gun's breech could be erected.

The guns were mounted on Batignolles style rail mounts which consisted of five steel cross ties with integral spades which were driven between the railroad ties. On top of these steel ties, two heavy girders were laid parallel to the tracks which bolted to the ties. The girders were equipped with 12 sets of flanges that matched the same number on the carriage. When the carriage was over the flanges the carriage was fastened to the girders and the ties. Once the platform was ready, the piece could be anchored in minutes and ready to fire. The ties and girders supported the weight of the carriage and absorbed the gun's recoil, the track did not have to be reinforced.[1] A pit was also dug under the gun's breech and lined with a steel box to allow the gun to recoil at high angles of elevation, which took about 3 hours. The Batignolles mounts also employed car-traverse which allowed for limited traverse by shifting the carriage on its bogies. When the United States entered the First World War a license to produce Berceau style rail mounts was negotiated with Batignolles and the carriage used by the 12-inch M1895 railway gun was similar.[1]