Direct Impingement vs. Piston Operated Firearms: A Basic Understanding

So, how does a semi-automatic firearm cycle? There are three main methods of operation that cycle the actions on semi-automatic and automatic firearms alike, all of which can be broken down into multiple sub-methods of operation. The three main categories are blow-back, direct impingement, and piston. We are going to look at the latter two: direct impingement and piston.

Cycle of Operations in a Firearm

To avoid the large debate that is sure to follow any article that talks about one of these two operating methods being better than the other, we will simply look at how the two operations cause a weapon to cycle. But first, we need a basic understanding of the cycle of operations in a firearm:

  • Loading is placing a live cartridge into the weapon so that it may feed fired.
  • Feeding is when the bolt pushes the round out of the magazine or off of the loading tray and begins to move it towards the chamber.
  • Chambering happens when the bolt pushes the cartridge into the rear opening of the barrel or the chamber.
  • Locking happens when the bolt locks in place against the chamber of to keep gasses from going anywhere but out the muzzle and to protect the shooter from dangerously high levels of pressure.
  • Firing occurs when the striker or firing pin ignites the round and the bullet travels down range.
  • Unlocking of the bolt from the chamber allows the bolt group to move rearward.
  • Extracting occurs as the bolt moves back and pulls the spent casing out of the chamber.
  • Ejecting is when the spent casing is thrown out of the weapon to make room for a fresh round.
  • Cocking resets the hammer, firing pin, or striker so that it may fire the next round.

This cycle is done manually with pump actions, lever actions, and bolt actions; to name a few. In a self-loading, or semi-automatic firearm, the weapon is capable of completing this entire cycle one time for every trigger pull without any other user input. So, how does it work?

Direct Impingement

Direct Impingement is a method of operation that uses the expanding gasses from the fired bullet to continue the cycle of operations. As the gasses travel down the barrel, they are directed through a small hole at the top of the barrel. This hole is connected to a tube that travels over the barrel and back towards the bolt group. At the end of the tube, the gas will press against the bolt group with enough force to push the bolt rearward, which will extract and eject the spent round. The weapon will be cocked and the bolt returned forward to feed another round and repeat the process up to the locking phase, where the shooter must pull the trigger to fire.

Piston Operated

like direct impingement, uses the expanded gasses of a fire round to complete the cycle of operations. There is a key difference though; the gas is not directed to the bolt face. Instead, the gas comes up from the barrel to meet the front end of a metal rod, known as a piston. The piston is connected to the bolt group, and as the piston is pushed back by the gas, the piston pushes the bolt back as the gas would in the above example, bringing the shooter back to the point of having a chambered and cocked firearm waiting for a trigger pull.

Both systems are well liked by many shooters, some with preference for one or the other. Each has their benefits and their drawbacks that can be debated until you are blue in the face. We will look at the basic claims of the two systems to get an idea what the end results are.

Gas

  • Lighter weapons, comparatively
  • Easier repairs
  • Less expensive

Piston

  • Runs cleaner as the gas is not dumped onto the bolt group or over the chamber
  • Runs cooler as there is more room for gas expansion
  • Often come with easy adjustments to change to gas flow for use with or without suppressors

As you can see, there are a few big differences between the two systems that can lead to long discussions as to which one is “better” and why. All I know is that I should have at least one of each.

Disclaimer: The content in this article is the opinion of the writer and does not necessarily reflect the policies or opinions of US Patriot Tactical.

Seth Belt

Seth grew up in Southern Arizona before joining the U.S. Navy. While serving in the Navy, Seth was an anti-narcotics operator and an anti-submarine operator for 5 years. He was lucky enough to travel to many of the Central and South American countries, as well as visiting many South East Asian nations and islands. One of Seth’s greatest joys from his time in the Navy was teaching new Sailors firearms education and safety. After leaving the Navy in 2010, Seth returned to Arizona and had a rough time learning how to be a civilian again, often working jobs that could barely pay the bills. After going to school, Seth became an Emergency Medical Technician in the Phoenix Valley, where he now lives with his wife and son. His areas of knowledge cover military, firearms, and emergency medicine.

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