Obusier de 400 Modèle 1915/1916

A mle 1915/1916 showing the details of its site preparation.

There were two models of howitzer produced the mle 1915 and mle 1916 which were differentiated by the gun barrels used:

• On July 22, 1915, at the request of the French GHQ, the Ministry of War ordered the construction of eight model 1915 howitzers and rail carriages from Saint Chamond. The barrels for these howitzers were a conversion of existing Canon de 340 mm (13 in) Modèle 1887 L/42 naval guns which were shortened to 25 calibers and bored out to 400 mm (16 in). The barrels came from the decommissioned pre-dreadnought battleship Brennus and the two Valmy-class coastal defense ships. Six barrels were converted in 1915, with three others following in early 1916. Of the nine barrels converted, eight were rail mounted while the ninth tube served as a test piece on the Gâvres firing range.[3]
• A second order was placed in January 1917 for four more model 1916 howitzers and rail carriages with three spare barrels, which were a conversion of Canon de 340 mm (13 in) Modèle 1912 L/45 naval guns which were shortened to 25 calibers and bored out to 400 mm. These came from the canceled Normandie-class dreadnought battleships.[3] Both the mle 1915 and mle 1916 had the same general characteristics and used the same projectiles. The theoretical barrel life was 1,000 rounds and in case of severe barrel wear it was planned to bore the barrels out again to 415 mm (16.3 in), but this was never implemented.[4]

Compare the original naval gun and the modified howitzer, the mle 1915/1916 barrel is 25% shorter, the largest projectile weighs nearly twice as much, the muzzle velocity is only 60 per cent of the original and the range of the projectile is slightly less despite firing at greater angles of elevation.

A mle 1915/1916 in firing position showing the recoil and earth anchor pits.

With the success of the mle 1915/1916 in mind, the French Army requested more howitzers be built but a lack of 340 mm barrels suitable for conversion forced them to find an alternative. Surplus Canon de 305 mm (12 in) Modèle 1887 L/45 naval guns were converted to 370 mm (15 in) using the same process of shortening the barrels, boring them out and building rail carriages for them. The resulting eight howitzers and four spare barrels were called the Obusier de 370 modèle 1915 and they served alongside the Obusier de 400 Modèle 1915/1916 in both world wars.

The railway carriage for the howitzers was called the affut à Berceau Saint-Chamond and was used on a number of different railway guns. The howitzers were mounted on rectangular steel bases, which were suspended on two railway bogies. The rear bogie had four axles, while the front bogie had six axles.[6] The number of axles was determined by the weight limit for European railways of 17 t (17 long tons) per axle and 17 × 10 = 170 t (170 long tons).[1] Since the barrels were naval guns that were not intended for use at high angles of elevation the trunnions were relocated relatively far forward and the howitzers were nose heavy which is why the front bogie had more axles than the rear.[6]

The howitzers had a Welin breech block and used separate loading bagged charges and projectiles. To load the howitzer the barrel was lowered to -8° and a loading ramp to the howitzer's breech could be erected. 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 ground platform anchoring and the rear bogie could be jacked up to help absorb recoil. The carriage used a car traversing mount so traverse was limited to a few degrees each side of the howitzer centerline.[6] The howitzers normally required two days of setup before firing, which consisted of reinforcing an area of track with wooden beams and digging a pit for the earth anchor and another for howitzers breech at angles of elevation more than 45°. There were also four jack stands that could be lowered onto the platform and fastened to the timbers to provide a stable firing platform.[7]

The first use of the mle 1915/1916 was during the Battle of Verdun. Two mle 1915/1916's (those of the 77th Battery of the 3rd Foot Artillery Regiment) were deployed 13 km (8.1 mi) to the south-west at Baleycourt to participate in the preparation for the French counteroffensive on October 21, 1916. It is estimated that 373 rounds of 370 mm and 400 mm were fired during this battle.[9] Fort Douaumont was the first to be hit and then Fort Vaux. On October 23, 1916, of the fifty or so shells striking the fort, six pierced the concrete, earth, and masonry defensive works. The first exploded in the infirmary, another in the hallway, three others smashed a casemate attached to the barracks and lastly, a munitions depot was hit, triggering a fire. Explosions and toxic gases forced the German garrison to evacuate the fort, allowing the French to reoccupy it the next day.[10]

The next use of the mle 1915/196 was during the artillery preparation of the Battle of the Somme. From June 30, 1916, they shelled the commune of Morcourt and reduced the fortified villages of Herbécourt, Estrées and Belloy-en-Santerre to ruins.[3]

In April 1917, the French offensive on the Chemin des Dames was accompanied by attacks north of Reims. The preparatory artillery bombardment fell on the forts of Brimont, Witry-lès-Reims and Berru, as well as tunnels and shelters under Mount Cornillet and Mont-sans-nom.[2] The three tunnels were up to 30 m (98 ft) underground and could accommodate up to three battalions which could be used for German counter-attacks. Aerial photographs showed the entrances and the ventilation shafts of this subterranean complex. This, supplemented by information provided by a prisoner, led to them being targeted. The first twelve shots targeted the tunnel's entrances to trap the Germans inside. On May 20, 1917, the ventilation shafts were bombarded and one of the shells, fired from Mourmelon-le-Petit penetrated 30 m (98 ft 5 in) of limestone and exploded in the gallery killing more than 400 German troops who died in the explosion, tunnel collapse and asphyxiation.[7]

At least two of these weapons were operated by the 53rd Coast Artillery Regiment, Coast Artillery Corps in France during World War I (almost all US Army-manned heavy and railway artillery in that war was operated by Coast Artillery units). The US Army built a one-off experimental 16-inch (406 mm) railway howitzer M1918 with some influence from the French design. Based on experience with these weapons, the Coast Artillery abandoned the low-angle disappearing carriage (after deploying one at Fort Michie, New York) for its next generation of large coast defense guns, the 16-inch gun M1919 and the 16-inch howitzer M1920. Barbette carriages with an elevation of 65 degrees were designed for the new weapons to allow plunging fire as enemy ships approached; this type of carriage was later used for new installations of 16-inch ex-Navy guns in World War II.[11][12]