What is Dwell Time? Gas Length vs. Barrel Length Explained

You've probably heard or saw discussions about why the relationship between the AR-15's gas length and barrel length -- which determines dwell time -- matters so much. But what even is dwell time? Let's cover it, and why it matters for the sake of your rifle's reliability.

What is Dwell Time?

Dwell time measures the amount of time the gas system in the AR-15 -- or any other semiautomatic, direct gas impingement-driven rifle -- is pressurized by the expanding gas created by the round's ignition in the chamber. In other words, the bullet and barrel trap all the expanding gas.

Credit: Bolt Carrier Group Blog

That pressurized gas wants to find a way to escape the barrel. But until the round exits the muzzle, the gas has only one other place to escape: Through the gas block and gas tube, and back into the bolt carrier group.

How is Dwell Time Measured?

Dwell time can be measured two ways. The first method measures the total time the round remains inside the barrel, between the moment of ignition and the moment it exits the muzzle. This is the method of measurement used by firearm designers and engineers, because it helps to calculate chamber and barrel pressures.

Dwell time can also be measured as the time the round spends in the barrel only after it passes the gas port and before it exits the muzzle. This method of measurement is better suited for selecting the appropriate gas tube length and barrel length, because it determines how much pressure reaches the gas system and bolt carrier group.

Why is Dwell Time Important?

The AR-15's gas system requires a certain amount of pressure to drive the bolt carrier group effectively. This is also true for other gas-operated semiautomatic rifles. An appropriate well time ensures enough pressure reaches the BCG via the gas tube, allowing it to overcome the spring tension of the buffer assembly and drive fully rearward. 

Appropriate gas pressure ensures your rifle operates reliably, ejecting and chambering rounds with reliable force, while still mitigating felt recoil.

What Happens if Dwell Time is Off?

If a rifle's dwell time is too short -- causing low gas pressure -- the bolt will "short stroke," and fail to fully actuate inside the receiver. This causes failures to eject and feed, resulting in jams. This may also cause the hammer to fail to re-cock, resulting in a dead trigger.

But if dwell time is too long -- resulting in over-pressurization -- you'll experience yet more issues: Bolts and springs can wear prematurely. The bolt may actuate too quickly, resulting in jams, and, in extreme cases, excessive shear loads caused by excess gas pressure can damage bolt lugs, hammers, trigger assemblies, and disconnectors. 

A common symptom of over-pressurization is high ejection force, which causes spent casings to slam against brass deflectors and fly out of receivers at excessive distances. Excessive fouling inside the bolt carrier group and upper receiver is another complication caused by high dwell time.

Does my AR-15 Have the Right Gas System?

Now that you've got a good understanding of how the relationship between barrel length and gas tube length affects the operation of your rifle, the real question to ask is, "Does my AR-15 have the right dwell time?"

Gas Length vs. Barrel Length for Optimal Dwell Time

Although endless other factors can dictate what gas system is optimal for your rifle, here's the basic guidelines to follow. Note that these guidelines apply to 5.56/.223-chambered rifles only. Other calibers use different gunpowder and case capacities that produce different gas pressures throughout the length of the barrel.

• 18"+ barrel: Rifle-length gas tube
• 14.5" to 18" barrel: Mid-length gas tube
• 10" to 14.5" barrel: Carbine-length gas tube

"Why do so many 16" ARs use carbine gas?"

The answer is simple: Gun manufacturers (and the U.S. military) want to ensure their rifles operate reliably, all the time. That means building rifles with gas systems that create higher dwell times and over-pressurization.

This setup guarantees the rifle will drive its bolt carrier group every time you pull the trigger, no matter what -- even if the bolt's incredibly dirty, the upper receiver and buffer tube's clogged with grime, or you're running cheap ammo with gritty brass and weak powder -- at the expense of slightly increased wear and more felt recoil.

This is why so many 16"-barreled AR-15s (and the military's 14.5"-barreled M4s) use carbine gas. Officially, though, a mid-length gas system is optimal for ARs using either of these barrel lengths. The U.S military even tested this, and found mid-length gas increased the performance and service life of their M4s.

Can I fix dwell time without changing gas tubes or barrels?"

Thankfully, yes. An adjustable gas block provides an easy fix for any rifle that's over-pressurized because of high dwell time. Simply use the block to restrict how much gas enters the tube. You'll know you're at the right pressure when you dial down the block until the bolt remains held to the rear after firing the last round in a magazine.

Under-pressurization from low dwell time can potentially be remedied by having a gunsmith widen the gas port on your barrel. This will allow more gas to enter the tube, increasing the pressure acting on the bolt carrier group. You can also test using a light buffer spring.

Dwell time is just one component of the AR-15 barrel.

Forging processes, twist rates, barrel length, and coatings all contribute to accuracy and reliably. Read our Guide to AR-15 Barrels and Twist Rates to learn more.