Gas System

Operation of the Gas System

(Model 1900, 1908, FSK15 and 1917)

This “gas” operating system is based on the “piston mode” of operation where the bolt is pushed backwards by a piston and a pushrod or “operating rod”. The Garand and the AK47 are prime examples. The other method is  “direct impingement” where the gas pressure pushes directly against the bolt. The original Swedish Ljungman AG42, the French MAS 49 and American M16 would be examples.

Once the cartridge is fired, the expanding  gas becomes the source of power for driving the piston and operating rod rearward. This gas is diverted down a 1 mm (.0445”) hole under the barrel, a few inches from the muzzle. This hole allows the gas into a tube wherein lies a piston which is driven back toward the rear of the gun. Attached to the piston is  push rod/operating rod, which pushes the bolt back removing and ejecting the spent round out of the chamber and away from the rifle. A large spring, compressed by the rearward movement of the operating rod now free of any resisting gas pressure, moves the bolt forward stripping a new round from the magazine and and pushes it forward into the empty chamber. The gun is now ready to fire.  Pulling the trigger starts the process all over again.

The system described above is no secret to modern shooters, most semi-auto’s utilize this method.  However, Mondragon’s idea was conceived in the late 1800’s and was probably the first.  A competing Mauser design of the day was recoil system and operated like the Luger pistol or the Italian Revelli-Terni rifle.

An unusual bias on the part of the German military at that time, was no gas pressure should be diverted from the barrel because of a perceived loss in velocity to the bullet. The German military was so adamant about this perceived loss that it refused to acknowledge the gas driven system until WWII.  Even then, the initial design incorporated a shroud around the barrel partially trapping the gas pressure after the bullet left the muzzle. i.e. G41
(Courtesy of German Military Rifles 1871-1945, pg. 181 )

A gas selector valve was incorporated at the front of the gas tube. It is a two-position lever. Gas modes are “on” or “off”.

The purpose of firing manually over autoloading is accuracy. The manual mode allows more accurate shooting than in autoload. (See Range Report section). In close combat situations, autoloading would be preferred.

On the Models 1915 and 1917, you will find a locking screw which does not permit the shooter to turn off the gas system.

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A standard 1908 is on the left and a FSK 1915 is on the right.  The second screw will not permit rotation of the gas valve.
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Gas valve removed from its gas tube.

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