The hydro-pneumatic recoil system was a gun recoil mechanism designed and developed by the French Army weapons engineers and was first used on the WW1 French 75mm field gun, known as Soixante-Quinze Mle 1897, in 1898. The hydro-pneumatic long recoil mechanism kept the gun’s trail and wheels perfectly still during the firing sequence. Because it did not need to be re-aimed after each shot, the French 75 could deliver fifteen rounds per minute on its target, either shrapnel or high-explosive. This system was used intensively by the French Army during World War I.

In the hydro-pneumatic recoil mechanism, the gun’s barrel slid back on rollers, which included a set at the muzzle, when the shot was fired. The barrel of the French 75mm field gun was attached near the breech to a piston rod extending into an oil-filled cylinder placed just underneath the gun. When the barrel recoiled, the piston was pulled back by the barrel’s recoil and thus pushed the oil through a small orifice and into a second cylinder placed underneath. That second cylinder contained a freely floating piston which separated the surging oil from a confined volume of compressed air.

During the barrel’s recoil the floating piston being pressed forward by the surging oil, compressed the air even further inside the confined volume. This action absorbed the recoil progressively as the internal air pressure rose and, at the end of recoil, generated a strong but decreasing back pressure that returned the gun forward to its original position. The smoothness of this system had no equal in 1897 and for at least another ten years. Each recoil cycle on the French 75, including the return forward, lasted about 2 seconds, thus permitting a maximum attainable firing rate of 30 rounds per minute.

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