Introduction to the AR-15 Gas System

Intro to the Direct Impingement Gas System

We’ll begin our discussion of the AR-15 gas system with a brief review of how the gun operates. The following diagram is a lot to look at. By the time we finish this series of articles, you will understand everything that is happening in the diagram, and where things go wrong.

The sequence begins with a live round in the chamber and the bolt locked.
The hammer strikes the firing pin, the firing pin strikes the primer, the primer ignites the powder in the cartridge. The pressure builds in the cartridge until the bullet is dislodged from the neck of the case. The bullet is driven onto the lands and down the bore.
As the tail of the bullet passes the gas port, gas floods into the gas system. Pressure accumulates in the BCG. This pressure pushes the bolt forward to relieve pressure between the bolt lugs and barrel extension.
The gas pressure drives the bolt carrier rearward. As the carrier moves, the cam pin rides in a cam pin track cut in the carrier. This causes the bolt to rotate and unlock from the barrel extension.
As the bullet leaves the muzzle, the gas pressure in the bore and gas system drops to ambient pressure. The entire bolt carrier group begins moving to the rear of the gun. The extractor pulls the spent case out of the chamber.
The BCG cycles to the rear of the receiver extension against the mass of the buffer and the tension of the buffer spring. The ejector flings the spent case out of the gun through the ejection port. The kinetic energy of the BCG and buffer is absorbed by the buffer spring until they come to a stop.
The loaded spring slows the mass to a stop and then drives it back forward. As the bolt passes over the magazine, it strips another round and drives it over the feed ramps and into the chamber.
As the bolt meets the barrel extension, the bolt comes screeching to a halt while the carrier continues to drive forward.
As the carrier continues forward, the bolt does not. This causes the cam pin to travel in the cam pin track, which causes the bolt to rotate and lock into the barrel extension.
The sequence ends with another live round in the chamber, ready for the next cycle.

The first half of this sequence is critical to the proper function of the AR. In our review of the AR-15 gas system, we will spend a lot of time exploring these events and interfaces in great detail. We will also explain how dysfunction of the gas system is responsible for many malfunctions in the AR, especially those that happen in the latter half of the sequence above.

It's All About the Gas

If it isn’t obvious by now, the operating system of the direct impingement (DI) GAS-operated AR relies on gas. The burning powder in a fired cartridge generates high-pressure gas to drive the bullet through the bore of the barrel and downrange. The gas system harnesses a bit of that power to cycle the gun.

What is the Gas System?

The AR-15 gas system is comprised of the gas block, gas tube, and several components of the bolt carrier group. The fluid path created by these components uses the gas harnessed at the gas port to cycle the gun. The diagram below illustrates the fluid path (in red) that drives the system. Each component that touches this fluid path plays an important role in the process.

The sequence of events within this fluid path that leads to the cycling of the gun is described below:

As the bullet travels down the bore of the barrel and the tail of the bullet passes the gas port, gas flows into the gas port.
The gas flows through the gas port and into the gas block.
The gas flows through the gas block and into the gas tube.
The gas flows through the gas tube and into the gas key.
The gas flows through the gas key and into the carrier.
Inside the carrier, the gas accumulates in a piston chamber formed by three components: the carrier, the bolt tail, and the gas rings.
As the pressure builds inside the piston chamber, the pressure pushes forward on the bolt, countering the pressure inside the chamber of the barrel and relieving some of the rearward force on the bolt lugs against the barrel extension.
The force from the gas pushes the carrier assembly rearward, causing the cam pin to ride in the cam pin track cut in the carrier, which forces the bolt to rotate out of the locked position.
As the carrier moves rearward, the bolt moves forward (relative to the carrier). As this happens, the gas rings slide past vents drilled in the right side of the carrier. As these vents are exposed to the piston chamber, the gas is released and exhausts out of the ejection port.
As the carrier moves rearward, the gas key and gas tube separate. The remaining gas in the gas system is vented into the upper receiver.
As the bullet exits the muzzle, the pressure in the bore drops to ambient pressure.

Each of these components and interfaces is important. We will revisit them individually in subsequent articles.

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