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Gas Operated

Home > Propellants, Firearms, and Ammunition Development > Evolution of Firearms > Repeating Firearms > Gas Operated

A discharging cartridge produces a large volume of hot gas that can be harnessed to do work. Just as burning gasoline pushes the piston of an internal combustion engine, expanding gas from a cartridge can be used to drive a piston attached to the repeating mechanism.

U.S. M1 Garand

Image courtesy of Forensic Technology WAI Inc.

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In gas operation, gas from the barrel is redirected 180 degrees to do work in the direction of the repeating mechanism. The gas may push on an internal piston (U.S. M1 Garand) or an external ring (Remington Model 1100 shotgun).

The vast majority of gas-operated firearms have ported barrels to bleed off the gas. The size of the ports depends on the following factors:

  • The position down the barrel
  • The operating pressure of the cartridge
  • The rate of pressure decrease as the bullet moves through the barrel

The actual gas piston can use either a short or long stroke. The gas enters a small chamber under the barrel; one wall of the chamber is the piston. The piston moves rapidly rearward and impacts the end of the rod attached to the bolt. The impact unlocks the action, and the recoil of the cartridge pushes the bolt open.


Blowback is a firearm design that directly utilizes the breech pressure exerted on the head of the cartridge case to actuate the mechanism. Blowback operation provides a high rate of fire as a balance for the commonly used, less powerful pistol cartridges.

Heckler & Koch MP5A3

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Most submachine guns function by blowback, although a few locked breech designs exist (Heckler & Koch MP5). Many low-cost submachine guns fire from an open bolt that has a fixed firing pin, which never retracts as in other gun classes.

The firing sequence of most submachine guns occurs as follows:

  • The trigger mechanism holds the cocked bolt back, usually behind a box magazine.
  • Pulling the trigger releases the spring-loaded bolt, which travels forward.
  • As the bolt travels, it strips the top cartridge off the magazine and feeds it into the chamber.
  • As the bolt closes, the cartridge fires, driving the bolt back to repeat the cycle.

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