I’ll be as clear as possible on this, gas-piston operated firearms are auto loading guns that use the gases generated by the explosion within the bullet cartridge to cycle the action.  There are multiple steps within this cycle and this can be found in many semi-auto, full-auto and selective fire firearms.  Semi-auto guns fire one round for each pull of the trigger.  Full-auto only firearms will continuously fire rounds as long as the trigger is held down, or until the magazine runs out of cartridges.  Selective fire firearms, such as assault rifles, can fire in both semi-auto or full-auto mode with usually the flip of a switch.  I’m going to describe closed bolt gas operated actions as opposed to open bolt.  Closed bolt guns start the cycle with the bolt in the forward position.  Open bolt guns fire with the bolt starting at the rearward position.

     There are various key components that be found within a gas-piston auto loading action.  The breech is the opening in the side of the gun where spent cartridges are expelled.  Some firearms can be loaded through the breech as well.  These important parts are:

-Trigger (we all know this one)

-Hammer (large metal piece that is released by the pull of a trigger, used to strike the firing pin)

-Firing Pin (long metal rod within the bolt that hits the primer on the bullet cartridge)

-Bolt (major moving component that sits behind the cartridge and moves rearward to extract the cartridge from the chamber)

-Extractor (claw-like piece on the bolt that clamps onto the cartridge lip so it can be pulled out)

-Ejector (spring-like metal tab that flings the spent cartridge cases out of the breech)

-Barrel (where the bullets fly out of the gun…)

-Gas Cylinder (metal tube that collects gasses from the barrel)

-Piston (rod that sits in the gas cylinder and gets pushed back by the gasses)

     So here’s how it all works, after the magazine is inserted, the gun is cocked by pulling back the bolt handle  and safety is turned off:

-Trigger is pulled, releasing the hammer

-The hammer pivots and strikes the rear portion of the firing pin within the bolt

-The firing pin hits the primer and ignites the powder, this then explodes sending the bullet down the barrel

-The gasses traveling behind the bullet and partially forced up through the gas cylinder

-The gasses push the gas piston back, which is connected to the bolt, therefore forcing the bolt backwards

-The extractor pulls the spent cartridge out of the chamber as the bolt travels back

-As the bolt goes back, it pushes the hammer back down, where it locks into place on the trigger’s sear

-The empty cartridge is forced out of the breech by the ejector once the bolt is fully rearward

-A large spring pushes the bolt forward, and pushes one new cartridge out of the magazine, into the chamber

     Now the cycle is ready to operate again.  In semi-auto guns, a special mechanism ensures that the hammer catches the sear after each shot so that the trigger cannot just be held down to fire rapidly like a fully-automatic firearm can.  In selective fire guns, a lever is usually flipped to move a special sear in place to determine whether the firearm may fire in full-auto or semi-auto.