Got Gas? A guide to understanding the AR15 gas system

In Technical Know-How by Eric Wise4 Comments

There’s a lot of confusion in the shooting community about how the gas system in an AR works and affects the reliability of the gun. In this article we’ll look at how the AR gas system actually functions and how the different components of the system effect reliability and shootability.

Imagine yourself at a match. You’ve loaded a magazine and charged your rifle. You’re standing in the start box, give a nod and hear the beeeeep of the timer. On your first shot, you’re thinking about how your sight picture looked, finding the next target and then starting the trigger squeeze for the next shot. But in that split second between shots, there’s a lot happening inside the rifle.

When you fired that first shot of the match the brass case you loaded expanded against the sides of the chamber, holding it in place. Gas pressure built up inside the case and propelled the bullet down the barrel. In the top of your barrel is a hole called the gas port and the gas port is covered by either a low profile gas block (typically found under free float rails) or a fixed front sight post (more common in military type ARs). As the bullet passed the gas port, the gas tried to escape the barrel through the path of least resistance (through the gas block, down the gas tube and into the action).

The gas pressure flowed down the gas tube and into the gas key on top of your bolt carrier group. This caused the bolt carrier group to start unlocking from the chamber and moving backward. The bolt pulled the now-empty case from the chamber and ejected it as the bolt moved backward passed the ejection port.

As the bolt carrier group slowed down and cycled forward again, it stripped a round off of the top of the magazine and chambered it for your next shot.

So now that you know what happened when you pulled the trigger, you can better understand how all of the parts of the system work together.

As you can imagine, the closer the gas port is to the chamber, the higher the gas pressure is going to be as the gas flows through the port into the bolt carrier group (BCG). Common gas systems are referred to as (from shortest to longest) carbine, mid, intermediate and rifle lengths. The chamber pressure on a carbine gas system and a 16″ barrel can average around 25,000psi while a rifle gas system on an 18″ barrel can average around only 17,000psi, a reduction of over 30% in pressure. All of that extra pressure can stress the parts of your BCG and cause premature bolt failures, among other problems.

The “dwell time” is the amount of time the BCG is receiving gas pressure and is determined by the length of barrel between the gas port and the muzzle. This measurement can effect the reliability and shootability because it also determines how much gas is fed to the action.

If the BCG doesn’t receive enough gas, a couple of problems can occur. The BCG might not cycle far enough back to eject the empty case, it might eject the empty case but not cycle far enough back to strip a fresh round off of the magazine or the BCG might not lock to the rear on an empty magazine because it’s not cycling far enough back. These malfunctions are commonly referred to as “short stroking”. They can result from weak ammo (not developing enough gas pressure), the gas ports being too small (not enough of the available gas being sent to the BCG) or not enough dwell time (the BCG not receiving gas for long enough to cycle properly).

Sometimes the BCG can receive too much gas. This is most commonly a result of the gas port being too large (feeding the BCG too much of the available gas), the gas system length being too short (the gas pressure being too high as it flows to the BCG) or the dwell time being too long (the BCG being fed gas for longer than it needed). These problems are commonly referred to as a rifle being “over-gassed”. If a rifle is over gassed the BCG can be stressed from the excess pressure, causing premature wear and failure of the bolt. Over gassing is also a major cause of excessive recoil in ARs.

Now before the scoffing and mocking begins about my complaint of recoil from the lowly .223, let me explain. When most people think of recoil they remember the first time they shot their dad’s deer rifle as a kid or the think of sore shoulders after a long day hunting doves with a shotgun. Comparatively speaking ARs don’t “recoil” much, if at all. But in the context of competitive shooting, they do. You won’t feel the recoil on your shoulder but you’ll see it in your sights. Set up three targets within a few yards of each other at the end of a deep bay. As you fire at the first target, you’ll have ideally called the shot as a hit and started moving to acquire the second target. You’ll never feel the recoil of a .223 on your shoulder, but you’ll see the recoil as you try to transition to that second target. If you can eliminate as much recoil as possible from the gun, your sights won’t move off of the targets and your split time from target to target will be faster. On close targets (7-10 yards) you’ll likely never notice any difference between a carbine and rifle gas system. But on offhand shots at 50-100 yards you’ll definitely notice. And as we noticed in the match we hosted back in September, the ability to make fast shots offhand at 75 yards made huge differences in the final scores.

The buffer system is the last part of the gas system and is often overlooked. As the gas pressure causes the BCG to start moving back, the buffer and spring in your stock creates resistance to slow the BCG and then push it back into the chamber. If your buffer is too light the BCG will slam back very quickly, while a heavier buffer will slow the BCG and turn the quick punch of the recoil into a firm shove.

The most common type of gas system for competitive shooting is an 18″ barrel with rifle length gas. It’s reputation for being so smooth shooting comes from a larger distance between the chamber and gas port and a shorter distance from the gas port to the muzzle. This combination has proved to have enough gas pressure to be extremely reliable, but not so much that the recoil is disruptive to your sight picture. 16″ barrels with mid-length gas, when ported correctly, can be very reliable but still useful in a competitive setting. This is probably the ideal combo for a patrol officer wanting a “jack-of-all-trades” rifle for work and competition. For matches with short range, fast shooting I think you’ll start to see more 14.5″ barrels with mid-length gas being used in competition. I’ve had the opportunity to shoot a BCM 14.5″ mid-length upper and with an H2 buffer this set up is incredibly soft shooting and easy to keep on target.

To make sure that their rifles function with any ammo (from high velocity 5.56 to very weak training ammo) some manufacturers will give their barrels larger gas ports and lighter buffers. Solving an over gassed rifle at the gas port can be expensive and time consuming. But adding a heavier buffer can mitigate some of the “gassy-ness” and smooth everything out. Tuning the gas system of your AR can help you get accurate shots down range faster, which is the goal of any kind of shooting. But you’ve got to remember that all of the parts of the gas system work together and you can’t change one part of it without affecting everything else.

And really, that’s what building a competition gun is all about. The smoother you are in your draw, your movements and your transitions….the faster you’ll be. So do yourself a favor and make sure that your AR is running as smoothly as you are.


  1. I would rather have over gassed vs under gassed.

    For a gaming/competitive gun I would be willing to tweak the gas system until is was very smooth but close to not cycling properly. If it does not operate when you need it to then you lose a game and not your life.

    For a defense rifle, I would stay away from rifles with shorter dwell times, 14.5 middy and SBR lenghts (10-12inch) are going to possibly have issues with under powered ammo. A 14.5 inch carbine, 16 inch middy and 12inch rifle all have a dwell time of about 7.5 inches and they will cycle anything. A 16 inch carbine will as well but it has a 9.5inch dwell time so it will cycle harder.

    1. The gas in the piston area of the bolt and carrier does not push the BCG to the recoil position. The gas from the gas tube that pressurizes the bolt carrier group works to push the bolt and carrier apart. It cannot push the bolt forward more than a few “.001s”, and so what does move is the bolt carrier. When that happens the bolt is cammed to the unlock position and pressure in the chamber pushes the entire contraption to the rear. Residual chamber pressure does the work. The gas in the gas tube only unlocks the bolt, then pressure in the barrel cycles the action. Too long of a dwell time will short stroke the gun as too much pressure bleeds out of the muzzle.

  2. Cobraman. Too short of a dwell time results in short stroking. And it is the gas pressure inside the bolt carrier that pushes the carrier back, unlocking the bolt, but it is mainly the momentum of the carrier backwards that cycles the action. The bullet is usually out of the barrel and pressure in the camber usually dissipates enough to be considered negligible to the cycling of the action by the time the bolt unlocks. Slow motion footage of an action cycling will confirm this as you will see no gas exiting the chamber as the case is being extracted in most cases. Not saying a combo of short gas system and long barrel won’t result in that.

  3. Just built my first light weight AR with 16″ barrel, a Next Intent adjustable gas block, fixed stock A1 buffer tube, 3.7 oz buffer, V7 titanium BCG, Elftmannot trigger.

    I adjusted the gas block to lock back the BCG but on the second round the trigger does not seem to engage. (Nothing).
    Is it short stroking? Should allow less gas to leave at the gas block? Do I need a heavier buffer?


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