Gas System Variables
There are only a couple of variables that you can control with the gas system. But those variables are very impactful. Understanding the effects of certain specifications makes
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.
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.
Gas System Length vs. Port Pressure
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 pressure more space to fill. So, as the bullet travels away from the chamber, the bore/chamber pressure decreases.
Accordingly, a gas port placed closer to the chamber will experience higher peak port pressures (the maximum pressure measured at the gas port). A gas port farther from the chamber will experience lower peak port pressures. As such, gas system length is an important variable in the design and function of the gas system.
The following table illustrates the empirical port pressures of M193 ammo, using data as reported by AR15Barrels.com.
Higher pressure at the gas port forces extra gas through the gas system. Assuming the same gas port diameter without restriction, a pistol length gas system will see much higher gas system pressure and flow than a rifle length gas system.
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 System Length vs. Dwell Time
Dwell time refers to the time during which the gas system is pressurized, before the bore pressure drops to atmospheric pressure. Practically speaking, this is the time that it takes the tail of the bullet to travel from the gas port to the muzzle.
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.
Gas System Length vs. Bolt Lock Time
Bolt lock time is the amount of time after the primer is ignited before the bolt rotates and unlocks from the barrel extension. The graph below (using AR15Barrel.com data) shows 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).
When we overlay the time chart with the pressure chart, we can see that shorter gas systems are not only pressurized sooner, but to substantially higher pressures. And this can be a problem.
When the firing pin strikes the primer, it sets off a violent chain reaction. Pressures build in the cartridge until the bullet is dislodged from the case neck and sent spiraling down the bore. At such high pressures, the case swells inside the chamber. The higher the chamber pressure, the more the case expands against the inner walls of the chamber and the less it wants to move. As the projectile moves down the bore toward the muzzle, the chamber pressure drops, reducing the outward pressure on the case.
Why is this relevant? If you try to yank the case out of the chamber too soon, a couple of things will happen:
- The case will still be expanded in the chamber, and it will take significantly more force to yank it out. If you pull hard enough on the rim, you could tear the rim off the case.
- The pressure against the bolt face will be significantly higher. As explained in the Gas System Intro, the components of the bolt carrier group form a gas piston and the pressurization of this piston chamber pushes the bolt forward, counteracting the chamber pressure and allowing the bolt to rotate with minimal shear against the barrel extension. If the chamber pressure has not dissipated, the pressure that builds inside the bolt carrier will not be able to relieve the pressure between the bolt lugs and the barrel extension lugs, but the bolt will still try to rotate. This added resistance creates a few issues:
- This will increase the shear stress on the bolt lugs and lead to fracture and premature failure.
- The cam pin bore in the bolt will experience a lot of stress as the carrier tries to move rearward before the bolt is unlocked. The cam pin bore happens to be the weakest point on the bolt and the metal will stretch and quickly fail under this stress.
- The added resistance 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.
The longer the bolt unlock can be delayed (without causing dwell and pressurization issues), the easier the bolt will be able to extract the spent case.
So, from this 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).
In Summary:
- The AR gas system needs enough gas to operate, but not too much.
- The shorter the gas system, the quicker the gas system pressurizes.
- The shorter the gas system, the higher the peak gas port pressure.
- The shorter the gas system, the more stress on the gun, as a whole.
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