Actions: Gas Operated: Long Stroke Piston

In our last post, we studied the gas operated, short stroke piston action. During this post, we will study the other gas operated action that uses pistons, the long stroke piston. This is a mechanism that was used by the American M1 Garand rifle (not the carbine, which uses a short stroke piston), the Soviet AK-47 family (and its various derivatives made in other countries, such as the Israeli Galil, Finnish Valmet etc.), the Belgian FN FAL assault rifle etc.

Like the short stroke piston we've already studied, when the cartridge is fired, hot high pressure gases are generated which push the bullet out of the barrel. Most of these hot gases also escape behind the bullet, but some of it is tapped out and used to operate the extraction and reloading mechanism to eject the spent cartridge and load the next cartridge.

Click to enlarge image

Original file licensed under GNU Free Documentation License version 1.2 by user Thuringius, on en.wikipedia.org

In the above image, (1) is the gas port from which a portion of the hot gases leaving the barrel are tapped. (2) is the head of the piston and (3) is the piston rod. (4) is the bolt that holds the cartridge in place in the breech and (5) is the bolt carrier that moves the bolt. (6) is the return spring.

Initially, the bolt holds the cartridge in place in the breech chamber. When the trigger is pulled, it releases the hammer (not shown in the image above), which strikes the base of the firing pin (not shown in the image above). The other end of the firing pin strikes the base of the cartridge, which detonates its primer. The primer in turn, causes the propellant of the cartridge to burn and generate hot, high pressure gases. The generated gases expand and push the bullet out of the barrel.

Click to enlarge image

Original file licensed under GNU Free Documentation License version 1.2 by user Thuringius, on en.wikipedia.org

As the bullet is pushed out of the barrel, some of the high pressure gases generated by the exploding cartridge are tapped via the port (1) and enter the tube containing the piston. The high pressure gases act upon the piston head (2) and push it backwards. Since the piston rod (3) is connected to bolt carrier (5), they move backwards, compressing the return spring (6). At the same time, the bolt carrier (5) also picks up bolt (4) and moves it backward at the same time. The bolt has an extraction pin that picks up the spent cartridge case and pulls it out of the breech. As the spent cartridge moves backwards, it is ejected out of the gun via a side port. The piston, bolt carrier and bolt continue to move backwards and re-cock the gun. Meanwhile the high pressure gases acting on the piston head (2) are dissipated via a vent hole in the tube. When the bolt carrier moves backwards to its maximum, the compressed return spring (6) pushes it forwards again. This moves the piston, bolt and bolt carrier forward again and on the way forward, they pick up a new cartridge from the magazine and push it into the breech chamber. Now the weapon is ready to fire again.

In the above animation, you can observe clearly how the mechanism works (including the hammer and firing pin, which were not present in the images above).

The main difference between the long stroke piston action and the short stroke piston action is that in a short stroke action, the piston moves back violently only a short distance (usually less than its own diameter) and then it is stopped by a projection. The bolt carrier then separates from the piston and continues to move backwards due to momentum. As a result of this, a short stroke piston action generally taps gases from closer to the breech (closed end of the barrel). In a long stroke action, the piston moves a distance greater than the length of the cartridge. In fact, the piston stays attached to the bolt carrier throughout the operation. This causes the piston to have greater dwell time and therefore, long stroke piston weapons generally tap their gases closer to the front end of the barrel (the muzzle). It also means that the operating parts are longer and therefore heavier than that of a short stroke piston action.

The advantages of this type of action are many. Like the short stroke piston action, the hot, high pressure gases only act upon the piston head and therefore, the other operating parts (the bolt, bolt carrier, trigger assembly, hammer, firing pin, return spring etc.) are not exposed to the hot gases or the dirty residue at all. These weapons can therefore accept a wide variety of cartridges using different quality propellants, since the dirty residue from low-quality propellants does not have a chance to contact most of the key operating parts of this action. This means a more reliable action and ease of maintenance of the weapon. The extra weight of the piston rod acting on the bolt carrier means more positive extraction, chambering and locking of the cartridges.

There are also some disadvantages. The main one is that the mass of the moving parts alter the center of gravity of the weapon and thereby make it harder to keep the weapon pointed on target, especially when firing in automatic mode. The abrupt stops and starts of the bolt carrier, piston and bolt at either end of the cycle also contribute to the shaking. Also, because the operating parts are longer, the overall weight of weapons that use this action are a bit heavier than short stroke piston actions.