Gas System Variables and Effects

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.

Inputs

Input: Gas System Length

The gas system length refers to the distance from the chamber (usually measured from the back end of the barrel extension) to the gas port. This distance defines the length of the gas tube, which reaches from the gas block into the upper receiver and into the gas key. There are four standard nominal gas system lengths:

Nominal Length	Gas System Length	Gas Tube Length
Pistol	~4”	6.625-6.75”
Carbine	~7”	9.25-9.8”
Mid	~9”	11.25-11.75”
Rifle	~12”	15.125”

There are some other gas system lengths, including Intermediate, Rifle+2 (inches) and some oddball lengths. But these four are the ones you are most likely to encounter.

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

Input: 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.

Input: 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 our next article.

Input: 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.

Input: 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.

Effects and Symptoms

Effect: 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).

Effect: 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.

Effect: 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.

Effect: 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.

Effect: 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.

Effect: 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.

Effect: 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.

Effect: Locked Bolt Time and Reciprocating Mass Movement

Locked Bolt Time is the amount of time between primer ignition and unlocking of the bolt from the barrel extension.

At this point, we have established that a longer gas system takes more time to pressurize. This time reflects the composite effects of:

Longer gas port distance
Higher pressurized bore volume
Lower gas port pressure
Longer gas system fluid path
Higher gas system volume

We know that it is gas system pressurization that results in unlocking of the BCG. So, it is logical to conclude that the time it takes to pressure the gas system will directly impact the time it takes to unlock the BCG. The longer the gas system, the more time to unlock the bolt. The shorter the gas system, the less time to unlock the bolt.

But what does that really mean?

First off, the most obvious consequence of earlier unlocking is that the reciprocating mass (BCG and buffer) begins moving in less time. A moving part in the gun moves the gun, itself. If the gun begins moving before the bullet leaves the muzzle, it can have a negative impact on accuracy and precision of the shot. The reciprocating mass is not an insignificant weight to be flying around inside the gun, so the effects of this mass moving earlier can be noticeable.

The second consequence of earlier unlocking of the bolt is a bit more subtle.

In the Gas System Intro article, we saw that the chamber pressure causes case to swell in the chamber.

The swelling of the case in the chamber causes the case to press against the bolt face, pushing it rearward. This causes the bolt lugs to press against the star chamber lugs. The amount of pressure between these lugs is proportional to the chamber pressure. The higher the chamber pressure, the more significant the rearward force.

As we have established, the peak port pressure decreases with a longer gas system. The gas pressure in the chamber is approximately equal to the pressure at the gas port. So, as the projectile moves down the bore toward the muzzle, the chamber pressure drops, reducing the outward pressure from within the case.

Why is this relevant? There are a couple of reasons.

The earlier in the firing sequence, the higher the pressure inside the case, and the harder it will press against the walls of the chamber. The harder it presses against the walls of the chamber, the harder it will be to pull the case out of the chamber. If you pull hard enough, you could tear the rim off the case, among other things.

The earlier in the firing sequence, the higher the pressure inside the case, and the harder it will press against the bolt face. The harder it presses against the bolt face, the harder the bolt lugs will press against the star chamber lugs, and the harder it will be to rotate the bolt. The added resistance creates a few issues:

It will increase the shear stress on the bolt lugs (and the star chamber lugs) and lead to excessive wear and premature fracture.
It will increase the tension applied to the bolt by the cam pin, which will lead to stretching of the bolt and premature fracture.
It will also put more stress on the cam pin and the cam pin track in the bolt carrier, leading to unusual wear and potential failure.

So, the more the bolt unlock can be delayed (without causing dwell and pressurization issues), the easier the bolt will be able to extract the spent case and the easier the bolt will be able to rotate in the star chamber.

We know that a shorter gas system pressurizes earlier and experiences higher pressures than a longer one. Together with our assessment of Locked Bolt Time, we can gather that the shorter the gas system, the more stress and damage is done to the firearm, as a system. Whether you experience the mutilated cases or not, a shorter gas system will absolutely shorten the life of the bolt. A bolt in a carbine length gas system will fail in ¼ the time of a bolt in a mid length system (yes, it will fail four times faster).

Effect: Reciprocating Mass Velocity and Momentum

As the pressure chamber in the BCG is pressurized, the carrier is driven rearward to begin the reciprocating cycle. The faster and more forcefully the BCG is pressurized, the more violent the force exerted on the carrier to drive it rearward is. So the higher the pressure delivered to the BCG, the faster the BCG (and buffer) will be driven rearward. Coupled with the weight of the reciprocating mass, this will have a proportional effect on the momentum of the reciprocating mass.

We need a sufficient impulse to be delivered to the BCG. If the pressure delivered to the BCG is too low, the impulse will not be sufficient to drive the reciprocating mass rearward with enough force to fully cycle the gun. The result will be either a failure-to-extract (if the bolt doesn’t travel far enough back for the brass to clear the ejection port) or a failure-to-feed (either the round won’t be stripped, or the bolt will drag it out of the magazine by the body instead of by pushing it by the rim).

However, too much of an impulse can have negative effects too.

First, if your bolt carrier accelerates faster, it will extract the spent case more forcibly. If the carrier velocity is too high, the brass will bounce hard off of the shell deflector. This will present itself as brass ejecting between 12:00 and 2:30 and dented cases, among other things.

Second, if the reciprocating mass has more momentum, it will take more to stop it (F = m * v). If the momentum of the reciprocating mass exceeds the buffering capacity of the buffer spring, the buffer bumper will slam into the back of the receiver extension, and this will contribute to a sharper felt recoil and mushrooming of the buffer bumper. Even if you’re spring is stiff enough to absorb the energy, you will still feel a stronger recoil impulse (Newton’s Third Law of Motion).

Third, your BCG can cycle too fast. If the reciprocating mass cycles too fast, the bolt will travel the rearward stroke and start moving forward earlier. This decreases the cycle time. It is nice to have the bolt get through its cycle faster so you can be ready for the next shot sooner. But things aren’t that simple. If the magazine spring has not had enough time to present the next round to the feed lips, you may encounter a failure-to-feed (either the round won’t be stripped, or the bolt will drag it out of the magazine by the body instead of by pushing it by the rim).

Effect: 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.

The industry standard for dwell time is between 0.18 and 0.20 milliseconds. This is generally optimal, assuming an appropriately sized gas port (shorter gas system = smaller gas port), standard mass carrier, and a reasonable buffer and spring combination. For an efficient gas system (which puts more of the gas into the BCG rather than losing it at loose junctions), you can get away with less dwell time. We will discuss gas system efficiency in a subsequent article.

Once you get down below 0.10 milliseconds, even the most efficient gas systems will be under-gassed.

When you get much above 0.21 milliseconds, most systems will be over-gassed.

The dwell time heatmap, below, helps us find the best combination of gas system length and barrel length. This heatmap is based on data generated by AR15Barrels.com using M193 ball ammo (note that this data is only accurate for the M193 cartridge).

Please note that this evaluation assumes an appropriately sized gas port for the stated system length, use of standard weight BCG, buffer, and spring, and an unsuppressed firearm.

Please note that pistol length gas systems are never ideal. The extreme port pressure amplifies the effects of the dwell time and creates its own set of issues. If you choose a pistol length gas system, it will be hard to tune and you will likely experience excessive recoil, wear, and fouling, even if it cycles acceptably.

The following graph shows the data a different way. From the data provided by AR15Barrels.com, we can calculate a linear trend using least squares analysis, and from this, calculate the ideal barrel length, based on the parameters of the testing (e.g. actual gas system dimension, cartridge, etc.). For example, the ideal barrel length for a mid-length gas system firing M193 ammo is between 16.06” and 16.78”.

Note that when you add a suppressor, a finely tuned gas system will be over-gassed. The back pressure from the suppressor essentially extends the dwell time.