In March 1918 a Commission was established to study guns which were très longue portée(TLP) - (very long range). The commission was motivated not only by the Willemgeschütze guns which commenced firing on Paris on 21 March 1918 but also the German 35.5cm and 38cm battleship guns which had considerably outranged the best French guns since 1916. The deliberations of the commission were quite brief and orders were quickly placed with Saint-Chamond and Schneider for TLP guns. In June 1918 tests were conducted with 37mm L/100 and L/160 guns to evaluate the internal ballistics of very long barrels. In late 1918 an experimental 145mm L/60 gun fired 80 rounds up to 1170 m/sec muzzle velocity before the barrel was worn out. By the time of the Armistice in November 1918 nothing concrete had been achieved with the full sized TLP guns although there were a large number of projects under investigation. The projects were rationalised after the Armistice and five 340mm L/45 Mle 1912 barrels were allocated to the TLP projects from the barrels constructed for the cancelled Normandie-class battleships.
The Schneider TLP projects used the carriage developed for nine 340mm Mle 1912 railway guns delivered in 1919. This weighed 270 tons and used the wooden block/rail recoil absorption system similar to that used on many of Schneider's WW1 railway guns. An obvious problem with the Schneider carriage was the lack of any traverse so it could only be fired from rails laid in a curve oriented with the target. The Schneider concept was to sleeve down the 340mm barrels to 240mm, 210mm and 224mm calibres in a similar fashion to the Willemgeschütze. Three different TLP guns were produced in great secrecy from 1920 to 1929 - the French were concerned about the reaction of Britain to the development of guns which could shell the Channel ports from France. The projects progressed quite slowly because of severely reduced defence spending after WW1.
This gun was a 240mm calibre sleeve in a 340mm barrel tested in 1924. The results were disappointing, the max. range achieved was around 50,000m at 37º elevation. The 170kg shells for the gun had a single band of engraved rifling in a similar fashion to the German Willemgeschütze. The projectiles in this, and other long range guns, experienced extreme accelerations on firing, so conventional copper driving bands could not be used since they would have been torn off on firing. The German solution, copied by Schneider, was to use a driving band engraved with a mirror image of the rifling. When loaded the projectile's driving band was carefully engaged with the gun's rifling. An obvious problem for the TLP project was to find a sufficiently long gun range to test these guns. The solution was to base the guns at St-Pierre-Quiberon, a peninsula in Southern Brittany and fire westwards over the Atlantic. A series of observing stations on the southern Brittany coast was maintained to observe the shots. There also was an underwater microphone network to record the fall of shot.
The 210mm gun was tested in 1921 and achieved a muzzle velocity of 1339 m/sec but the max. range achieved was poor due to the shells tumbling in flight. The 210mm barrel was found to wear very rapidly so the gun was returned to Schneider to be resleeved to 224mm calibre. Trials with 224mm gun in November 1927 achieved ranges of about 93,000m. This was confirmed in 1929 and 1930 trials. However, in June 1930 the gun experienced a massive breech overpressure which was found, on examination, to be due to severe wear of the rifling close to the breech producing high frictional forces on the shell. Schneider concluded after examination that the gun had been too badly stressed to be able to repair. The project appeared to be a dead end since the gun and carriage exceeded the French rail gauge and had to be dismantled by heavy equipment before transport.
The L/150 gun was an L/100 gun with an L/50 smooth bore extension. The carriage was revised so the gun laid flat on the carriage during transport but was raised by a hydraulic system so an elevation of 50º could be achieved. The gun was loaded at a 16.5° elevation. The smooth bore extension was carried on a separate car. The gun had a bridge with adjustable cables supporting the gun and the extension. It is thought that alignment of the smooth bore extension was achieved in a similar fashion to the German Willemgeschütze. The initial trials were carried out March 1929 when seven 150kg shells were fired at ranges from 71,000m to 107,000m. On November 1929 two modified 142kg shells were fired at 118,000 and 127,800m at a muzzle velocity of 1520 m/sec. Further trials in 1931 were conducted with 146 kg shells with two engraved bands1 ranges of 104,000m to 121,600m. After 35 shots the barrel was worn out and the gun was placed in storage. It was still in storage at the start of WW2 and although it was proposed to resleeve the gun to 220mm nothing was done before the German invasion in 1940.
The Saint-Chamond approach to a TLP gun was very different to Schneider's. Rather than reusing an existing railway gun carriage St-Chamond designed a new carriage with a central gun platform which could be rotated 360° (Tous Azimut). The gun barrel retained the usual Naval cradle and recoil cylinders so the preparation for firing seems to have been limited to swinging out the outriggers which prevented the carriage from toppling when firing across the line of the rail track. The St-Chamond project was approved in Nov. 1918 and the sole prototype tested in 1926. The 240mm L/51 gun achieved a max. range of 59,000m. However, although the St-Chamond project appears to have been closest to an operational long range gun, commercial imperatives intervened. In 1924 Schneider et Cie bought out FAMH (Saint-Chamond). The Saint-Chamond prototype TLP gun was sold to Japan in 1930 as a Schneider gun and accepted as Type 90 240mm Railway Gun. The Japanese Army deployed the railway gun to support the coastal defences guarding Tokyo Bay. In 1941 it was deployed to the Kwantung Army in Manchukuo (Manchuria) where it remained until 1945. During the Russian invasion of Manchuria the gun was briefly in action, it was destroyed and abandoned by the retreating Kwantung Army.