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What You Need to Know About Direct Impingement Gas Systems

Have you ever wondered exactly how AR-15s and other weapons get those bullets moving so quickly through the air?

There’s a fascinating internal structure in these guns called a gas impingement system that controls the power behind the bullet. This gas has an exceptional amount of power behind it, but it has to be expelled somewhere — namely, by pushing the bullet down the barrel of the weapon. However, there is a lot more going on behind the scenes for each shot than you may realize.

Faster, Less Reliable

It would be wonderful if these weapons became more reliable as the rate of bullet acceleration increased, but unfortunately — the opposite is true. As a direct impingement systems increases in power, the high-temperature gas causes condensation and the release of vaporized impurities that can clog the firing mechanism and cause the weapon to become inconsistent. Any weapon using a gas impingement system will need much more frequent and thorough cleaning to maintain a high degree of safety and accuracy.

Multiple Moving Parts

There are dozens of moving parts inside a gun, and all that happens in an instant when you pull the trigger. While the only external movement that you see is a cartridge being ejected and the bullet leaving the barrel, inside there’s a symphony of action that must all happen perfectly in alignment for the gun to fire. Direct impingement gas systems add a level of complexity to the process, which means that the weapons may not be quite as reliable as those using a simpler system to fire.

Some quick facts:

  • The gas surrounding the bullet is moving about 5x faster than the bullet when they both leave the barrel
  • Your bullet’s size should closely match the throat diameter of your weapon, or you risk having a wobbly bullet as it won’t travel smoothly down the barrel
  • Inconsistencies in the crown of the bullet can cause the bullet to go off track (more gas escaping on one side than the other)
  • Gas impingement systems not only cause the bullet to leave the barrel but also play a part in ejecting the cartridge case from the chamber
  • There is a slight compression of the cartridge case after the bullet has been expended — enough to allow the case to leave the chamber
  • Common Gas Systems

    There are a variety of barrel lengths that utilize gas impingement systems from a 16″ carbine that averages around 25,000 psi, up through mid-size, intermediate and rifle-length systems. The longer the barrel, the less pressure there is as it’s spread across a longer distance. Rifles may “only” average around 17,000 psi — still a significant amount of force being applied to move the bullet! As you can imagine, the systems with a great deal more pressure on them with the shorter barrels have a much higher likelihood of failure.

    While there are some challenges with gas direct impingement systems such as the extra cleaning needed and the slightly higher possibility of failures, it’s not surprising to see why they are so popular. The additional power that comes from this unique delivery mechanism provides a major boost to the speed of bullets.

    ~ Firearm Daily

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