What is Dwell Time? Why it Matters For Your AR's Gas System

There are a lot of AR-15s running the wrong gas system length, which can make your rifle, SBR, or pistol unreliable. The wrong gas system can can cause excessive recoil and increased wear, or it can cause short strokes, poor cycling, and frequent jams.

If your AR's suffering any of these issues, the most likely cause is a sub-optimal dwell time caused by using wrong gas system for your AR's barrel length, caliber, and general configuration.

What Does "Gas System Length" Mean?

The length of the AR's gas system directly affects dwell time, so it's important to first understand what it is. The gas system length refers to the distance of the barrel's gas port from the barrel extension. The further away the gas port is, the longer the gas system. There are four common lengths:

• Pistol Length (4")
• Carbine Length (7")
• Mid Length (9")
• Rifle Length (12")

Note that barrel length and gas length are independent from one another; the former doesn't dictate the latter. For example, a 16" barrel can have either a carbine- or mid-length gas system.

Also note that gas tube lengths differ from advertised gas port lengths. For example, the 4" pistol gas system uses a pistol gas tube that measures 6.75" long.

What is Dwell Time?

Dwell time measures how long it takes for a live round to travel down the barrel and exit the muzzle after it has passed the gas port. In other words, how long does the round dwell inside the barrel between the port and muzzle?

During this brief moment, the gas port is open but the muzzle is sealed by the bullet. With the bullet effectively plugging the end of the barrel, pressurized gas has only place to go: Into the port and gas block, through the gas tube and, ultimately, into the bolt carrier group.

Why is Dwell Time Important?

The AR's bolt carrier group requires a certain amount of gas pressure to function reliably. As gas travels through the carrier key and into the carrier itself, it forces the bolt to rotate and unlock from the chamber. That same pressure also forces the BCG rearward into the buffer tube. As it travels rearward, it performs a few key tasks, including ejecting the spent casing, resetting the hammer and trigger, picking up a fresh round from the magazine, and chambering it.

These key tasks are what allow the AR to function reliably as a semiautomatic weapon. Importantly, this cyclic action requires a certain amount of gas pressure -- too much gas (called over-pressurization or over-gassing) or too little gas (under-pressurization/under-gassing) throws the whole cycle off.

Over- and under-pressurization is directly caused by having an improper dwell time, which is the result of running the wrong gas system length.

What About Gas Port Diameter?

The diameter of the barrel's gas port also plays a key role in determing how much gas enters the gas block, tube, and bolt carrier group. The wider the port, the more gas flows into the upper receiver before the bullet exits the muzzle. The smaller the port, the less gas flows. With this in mind, it's important to note that gas port diameter can make your AR feel over- or under-gassed, even if you've selected what could otherwise be considered an optimal gas system length for your AR's setup.

Side Effects of Over-Gassing

An over-gassed AR-15 will produce high felt recoil. The bolt will actuate with excessive force, slamming into the rear of the buffer tube. The lugs on the bolt will also show signs of added wear, and the upper receiver can show signs of early wear as the BCG's hardened steel rides against the receiver's aluminum surfaces with excessive speed and force.

You can tell when an AR's over-gassed because of its shell casing ejection pattern. Casings will typically fly forward and to the right of the shooter, often at excessive distances. Casings may also display signs of shearing or damage from being driven into the chamber by the bolt with excessive force, too. You may notice brass deposits accumulating rapidly on the shell casing deflector.

Lastly, excess fouling will be present. This is caused by the excess gas forcing additional carbon fouling and debris into the bolt carrier group and upper receiver.

Side Effects of Under-Gassing

An under-gassed AR-15 will not cycle reliably. Quite often, the bolt will short stroke and fail to eject spent casings and chamber new rounds. This frequently leads to jams.

You may also find the bolt catch doesn't capture the bolt when the last round in the magazine is fired, requiring you to manually re-chamber the rifle after changing magazines by pulling on the charging handle.

In either case, improper pressurization of the gas system will lead to various issues that should be avoided. That's why it's so important to ensure your AR has the right gas setup to provide the proper dwell time.

What Gas System Should My AR-15 Have?

After many years of testing combinations of various barrel lengths and gas system lengths, it has been generally concluded that the ideal dwell time for 5.56 NATO and .223 Remington, when chambered in the AR-15, is between 0.12 and 0.18 milliseconds. This 0.06-millisecond variance in dwell time is meant to factor in that gas port sizes vary in diameter across the industry's different barrel manufacturers.

These are the ideal, standard, and sub-optimal gas system lengths for the AR-15's various barrel lengths, based on obtaining the optimal dwell times mentioned above:

Ideal Gas System Lengths

These gas and barrel length combos provide approximately 0.15 milliseconds of dwelltime, plus or minus a few milliseconds based on port diameter. These setups produce reliable cycling with the least felt recoil and the least carbon fouling relative to round count. These combos also work best when suppressed, as they help to keep blowback to a minimum.

• Pistol Length: 8" to 9" Barrel
• Carbine Length: 11.5" to 12.5"
• Mid Length: 14.5" to 16"
• Rifle Length: 18"

Standard Gas System Lengths

These gas and barrel length combinations produce slightly more felt recoil and carbon fouling with each round fired, but they aren't typically considered over-gassed. Dwell times are typically close to 0.18 milliseconds with these combinations.

Manufacturers typically choose these configurations when selling assembled rifles because the extra gas pressure helps to guarantee reliable cycling when the rifle is dirty, or when chambering cheap ammo.

• Pistol Length: 7.5"
• Carbine Length: 14.5"
• Mid Length: 16"
• Rifle Length: 20"

Over-Gassed (Excessive Dwell Time)

These gas and barrel length combinations yield as much as 0.21 milliseconds of dwell time (if not more), which will cause excessive felt recoil and rapid fouling. These combinations are also likely to introduce early wear and tear on the AR's receivers, fire controls, and bolt carrier group.

• Pistol Length: 10.5"
• Carbine Length: 16"
• Mid Length: 18"
• Rifle Length: 22"

Under-Gassed (Short Dwell Time)

These gas and barrel length combos produce too little dwell time -- some as short as 0.08 milliseconds -- which will typically result in insufficient gas pressure reaching the bolt carrier group, causing short strokes and jams. Another side effect of these minimal dwell times is excessive muzzle blast.

• Pistol Length: 7.5"
• Carbine Length: 10.5"
• Mid Length: 12.5"
• Rifle Length: 16"

How To Fix an Over-/Under-Gassed AR

Thankfully, fixing an AR with an over- or under-gassed AR is relatively easy. To fix an under-gassed AR, one only needs to increase the diameter of the gas port. This is done by drilling out the port -- we recommend letting a gunsmith perform this work.

(Is your gas block mis-aligned?)

If you're building your AR's upper and find it's short-stroking or seems under-gassed, it is possible the gas block isn't properly aligned with the gas port on the barrel. This can be remedied by dimpling the barrel to guarantee proper alignment of the block with the port. Here's how to do it.

An over-gassed AR can be easily tamed by installing a heavier buffer. Although this won't necessarily reduce carbon fouling, it will reduce felt recoil and the excessive wear and tear an over-gassed system otherwise causes.