Gas System Variables

There are only a couple of variables that you can control with the gas system. But those variables are very impactful. The series of effects from only a couple of variables is complex. The diagram below illustrates the relationships between the inputs and the sequential effects of those inputs.

Gas-dilocks: A balance of Force

The AR gas system relies on gas pressure and volume.

Pressure provides the impulse that drives the reciprocating mass (BCG and buffer) into action and cycles the gun. You need enough force to unlock the bolt and to drive the BCG hard enough to overcome the inertia of the reciprocating mass and the spring tension of the buffer spring.

The volume of gas flowing into the gas system sustains the force. You need to pressurize the system long enough to ensure sufficient volume to cycle the gun.

There are a few concepts and relationships that we need to discuss to understand optimization and dysfunction in the gas system. We will systematically dissect the variables and their impacts on the function and performance of the AR.

AR Gas System Inputs (Independent Variables)

Gas System Length

The gas system length refers to the distance from the chamber to the gas port. The length is typically expressed relative to the front of the barrel extension flange. This distance defines the length of the gas tube, which reaches from the gas block into the upper receiver and into the gas key.

The differences in the distance from the chamber to the gas port are extremely impactful to the operation of the AR system.

For more information about gas system length, check out our Barrel Deep Dive series.

Barrel Length

Changing the barrel length can have a lot of effects. When you change the length of the barrel relative to the length of the gas system, you change what happens after the bullet passes the gas port.

For more information about barrel length, check out our Barrel Deep Dive series.

Gas Port Size

The diameter of the gas port determines how much gas can flow into the gas system. This has a tremendous impact on the function of the gas system and the AR operating system, as a whole.

Gas System Efficiency

The efficiency of the gas system determines how much of an effect the gas flowing into the system actually has on the operation. The less efficient the gas system, the less of the gas that enters the system through the gas port will actually make it to the BCG. To achieve a certain amount of system pressure, flow, and force in a less efficient gas system, you need higher pressure and more flow at the gas port.

We will explore gas system efficiency more in another article.

Gas Block Design

Gas blocks come in a couple of flavors.

With a fixed gas block, you get what you get. Adjustment of the gas flow is not possible.

Adjustable gas blocks allow you to modulate the gas flow by constricting or venting the gas.

If your AR is equipped with an adjustable gas block, the setting will affect the gas flow into the gas system.

Reciprocating Mass Weight

While not an input to the gas system itself, the weight of the reciprocating mass determines how much force needs to be delivered by the gas system. The heavier the bolt carrier group and buffer, the more inertia they have. This means they are harder to get moving from a standstill, which means it takes more force to move them, which means we need a stronger impulse from the gas system.

Gas System Operating Outputs (Dependent Variables)

Port Pressurization Time

It makes sense that a bullet traveling at a given speed will take more time to travel a longer distance (t = ℓ/v). Therefore, it also makes sense that it will take more time before a longer gas system is exposed to pressure.

The graph below (using data from AR15Barrels.com) illustrates the logical relationship between distance and time (the longer the distance, the more time it takes for the bullet travel and for that point to pressurize).

Pressurized Bore Volume

The volume between the bolt face and the gas port obviously increases with a longer gas system. In fact, the change in bore volume due to a change in gas system length is proportional (V = π*r²*h). The difference in volume has a profound effect on the operation of the AR.

Peak Port Pressure and Flow Velocity

When the firing pin strikes the primer, the powder in the cartridge burns and generates crazy amounts of pressure (about 55,000 psi for a .223/5.56 in the appropriate chamber; for comparison, your car’s tires are typically inflated to 33-35 psi). This pressure propels the bullet out of the mouth of the case and down the bore of the barrel. As the bullet travels down the bore, the volume between the bullet and chamber increases, which gives the gas more space to fill. Per Boyle’s Law, P₁*V₁ = P₂*V₂; as you increase the volume, there is a proportional decrease in pressure. So, as the bullet travels away from the chamber, the bore/chamber pressure decreases.

Peak Port Pressure (the maximum pressure measured at the gas port) is a standard way to express the pressure entering the gas system. In line with our assessment above, a gas port placed closer to the chamber will experience higher peak port pressures. A gas port farther from the chamber will experience lower peak port pressures due to the higher bore volume.

The following table illustrates the empirical port pressures of M193 ammo, using data as reported by AR15Barrels.com.

When we overlay the peak pressure chart with the pressurization time chart, we can see that shorter gas systems are not only pressurized earlier, but to substantially higher pressures. And this can be a problem.

Peak Port Pressure is the consequence of the proximity of the gas port to the chamber, and it is extremely impactful to the function of the operating system.

In an unrestricted gas system, higher pressure at the gas port means:

The gas flows through the gas system more forcefully. All else being equal, higher gas pressure at the port will translate to higher velocity of the gas flowing through the gas system. The higher flow is due to a greater differential between the gas port and the pressure chamber in the bolt carrier (effectively, atmospheric pressure).
Higher flow velocity means that, once the gas system starts to pressurize, it will pressurize faster.
Because of the higher pressure and the higher flow velocity, the gas system is able to reach a higher pressure before it is vented to atmospheric pressure (i.e. before the bolt carrier begins to move).

Most barrel manufacturers somewhat compensate for the effect of gas system length on port pressure by adjusting the diameter of the gas port. A smaller gas port allows less gas into the gas system, and vice versa. However, the more extreme the port pressure, the greater the variation in performance due to ammo selection, powder temperature, etc.

Gas Tube Length/Volume

A longer gas system length translates into a longer bore distance between the chamber and gas port (which we’ve discussed) and a longer gas tube. As with the gas port length, a change in gas tube length affects the volume that it can hold. So an increase in gas tube length has a proportional effect on gas tube volume.

Gas System Volume

We just established that a longer gas system translates to a longer gas tube and higher gas tube volume. The gas tube is a component of the gas system and a significant component of the fluid path volume.

Logically, if a longer gas system results in a larger gas tube volume, then a longer the gas system results in a larger the total gas system volume.

Peak Gas System Pressure

Higher gas system volume means the gas system can hold more gas. We mentioned Boyle’s Law earlier in the context of bore volume, and the same principle applies to the gas system volume. When we increase gas system volume for a given amount of gas, we get a proportional decrease in gas system pressure.

The effect of gas system volume on gas system pressure is minor. But it does contribute to the observation that a shorter gas system pressurizes faster, more violently, and to higher pressures than a longer one.

Gas System Pressurization Time

Because of 1) a longer fluid path, and 2) a larger fluid path volume, a longer gas system will take longer to pressurize to the same pressure (assuming the same port pressure). Not only do we need more gas to fill the additional volume, but gas will have to travel farther, which will take more time at a given velocity.

The effect of gas system pressurization time due to gas system length is minor. However, it does contribute to the observation that a shorter gas system pressurizes faster, more violently, and to higher pressures than a longer one.

Dwell Time

Dwell time refers to the amount of time that the bullet is still in the bore after passing the gas port. It is the time during which the gas system is pressurized, before the bore pressure drops to atmospheric pressure.

For a barrel of a given length, the shorter the gas system length, the longer the dwell time; the longer the gas system length, the shorter the dwell time.

The dwell time influences how long the gas system is pressurized. As such, dwell time impacts how much of the gas (volume) makes its way through the gas system. As with pressure, there is a sweet spot for dwell time. Too short a dwell and there won’t be enough gas running through the gas system to operate the gun. Too long a dwell time and you will dump excessive amounts of gas into the gun.

We cover dwell time in a dedicated deep dive article.

Gas System Force

The ultimate goal and effect of the gas system variables is the amount of force conveyed to the bolt carrier to drive it into motion.

The gas system force is a function of the gas pressure, gas flow, and pressurization time.

The higher the gas system force, the earlier, faster, and harder the gun will cycle. We will cover the effects of gas system force in our next Deep Dive: Buffering System Deep Dive.