Actions: Gas Operation: Basics

In the last few posts, we've studied some modern firearm actions such as those that utilize the blowback principle and the recoil operated action. We will now study another family that is also heavily used in modern day weapons, the Gas Operated Action.

The basic principle of this family of actions is as follows: When a cartridge is fired, the propellant material burns and produces pressurized gas. The pressurized gas expands and pushes the bullet out of the front of the barrel. After the bullet is forced out of the barrel, the gas also leaves through the same exit. In a gas operated action, some of this pressurized gas is tapped out from the barrel via a port and used to operate a mechanism to remove the old cartridge, re-cock the weapon and chamber a new cartridge automatically.

The pressurized gas operates the mechanism in one of the following ways:

1. The pressurized gas operates upon a piston, which is forced to move by the expanding gases. The other end of the piston pushes the mechanism that ejects the old cartridge and loads a new one in. The two main types of piston actions are short and long stroke pistons. Piston operated mechanisms are used with weapons such as the M1 Garand, the AK-47 rifle family, the Bren LMG etc. Since the hot high-pressure gas operates upon the piston only, the rest of the mechanism is kept relatively cooler and cleaner, which makes the mechanism much more reliable to operate.
2. Gas trap: This works similar to long stroke piston mechanisms, but uses lower pressure gas to operate. This is a relatively rarer mechanism and was used with the German G41 rifle in World War 2, as well as some early Garands.
3. Direct impingement: Instead of the gas to push a piston, which in turn operates the mechanism, in a direct impingement action, the gas directly acts upon the bolt and carrier. Since there is no piston, this reduces the weight of the weapon and lowers the manufacturing cost. On the other hand, the firing mechanism becomes fouled more quickly and hot gases may also cause the different parts of the mechanism to expand at different rates and thereby lose accuracy. Lubricants in the mechanism also dries up and may cause malfunctions. This is the mechanism used by the M-16, AR-15 and M4 rifles.

Compared to blowback and recoil operated actions, which are used for lower powered pistols mostly, the gas operated action family is primarily used for automatic and semi-automatic rifles.

The first mention of a gas operated action was due to a Mexican General named Manuel Mondragon, who used it in a rifle design called the Mondragon rifle in 1887. It was the world's first semi-automatic and automatic rifle. Due to poor production facilities in Mexico at that time, he tried to get some US manufacturers to make the weapon, but could not garner enough interest at that time. However, he managed to convince the Swiss firm Schweizerische Industrie Gesellschaft (SIG) to manufacture this weapon for him. Excess stock of these weapons from SIG were bought up and used by Germany in World War I and World War II.

The next patent for a gas operated action was filed by the redoubtable American designer, John Moses Browning, who invented a gas operated action in 1889. Browning later licensed his design to Colt in 1892 and it was used to make weapons in a number of calibers.

In the next few days, we will study the various gas operated actions in some detail.