AR Dwell Time

What is 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.

Why is Dwell Time Important?

The dwell time directly affects 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. 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. The result can include short cycling, failure-extract, failure-to-eject, failure-to-feed, and other similar malfunctions.

Too long a dwell time and you will dump excessive amounts of gas into the gun. Once the bolt carrier moves to the rear, the gas pressure escapes in two places:

As the gas rings move past the vent holes in the carrier, the piston chamber vents through the ejection port.
As the gas key pulls away from the gas tube, the gas flows into the upper receiver.

How is Dwell Time Determined?

Dwell time can be measured, but it is not an easy task and requires precision instrumentation.

Calculating dwell time is a little bit challenging, because the bullet experiences non-uniform acceleration as it travels from chamber to muzzle. At the beginning of the journey down the bore, the bullet experiences a huge acceleration spike. The rate of acceleration decreases substantially as the bullet moves down the bore and then begins to level off. The farther from the bore, the less the change in acceleration per unit of distance.

You can indirectly estimate dwell time a couple of ways using readily available measurements, like muzzle velocity, gas length, and barrel length.

You can go bananas and use calculus to build a model based on the muzzle velocity while incrementally chopping the barrel down from a long barrel to a short barrel. We greatly appreciate our friends over at RifleShooter.com for doing this for many common cartridges. We use their data for many of our analyses. But we have an easier way to calculate dwell time.

You don’t need to go all Rain Man to get an accurate estimation of dwell time. Dwell time can be very precisely estimated based on muzzle velocity and dwell length. We have compared our short cut calculation to actual measurements, and the theoretical dwell time using muzzle velocity and dwell length is within a couple of microseconds of the measured dwell time (around 1% error; which is probably within the margin of error for the instruments and methods used to measure dwell time). We consider this close enough.

Our estimation of dwell time uses the t = x/v relationship. In its simplest form, we simply take the barrel length, subtract the gas port length, and divide by the muzzle velocity. This effectively give you dwell length over muzzle velocity, which approximates (very closely) the dwell time.

What is the Ideal Dwell Time?

Dwell time needs to be long enough to reliably cycle the gun, but short enough to not cause over-gassing and excessive amounts of gas flowing into the upper for no reason.

Standard dwell times typically assume an appropriately-sized gas port (shorter gas system = smaller gas port), standard cartridge load, standard mass carrier, and a reasonable buffer and spring combination.

Your build may benefit from playing around with dwell time within the acceptable range.

On the shorter end of the acceptable dwell time range, the recoil impulse is softer and the reciprocating mass moves closer to when the bullet exits the muzzle, both of which contribute to better accuracy and faster target reacquisition. However, the lower the recoil impulse, the less forceful and reliable the cycling will be. Shorter dwell time can cause under-gassed conditions and malfunctions if you don’t have an efficient gas system or an appropriate buffering system. Refer to our Gas System Deep Dive for more information about gas system efficiency. Refer to our Buffer System Deep Dive for more information about the buffering system.

On the longer end of the acceptable dwell time range, the system will cycle more forcefully, which is a huge benefit to reliability. However, the more forcefully the cycling, the harder it will recoil. A longer dwell time can disguise an inefficient gas system by compensating with a longer exposure time. However, longer dwell time can cause over-gassed conditions and malfunctions if you have an efficient gas system or an appropriate buffering system. Refer to our Gas System Deep Dive for more information about gas system efficiency. Refer to our Buffer System Deep Dive for more information about the buffering system.

As with everything in the gas system, ideal dwell time requires balance and consideration of the system, as a whole. If you are building a precision gun, you can err on the low end of acceptable dwell time as long as you ensure an efficient gas system; however, you will sacrifice muzzle velocity (which will reduce precision and range) if you opt for a shorter barrel. If you are building a combat carbine, we recommend you stay at the upper end of the dwell range (in addition to ensuring an efficient gas system). If you are building around a low pressure round (sub-sonic .300 AAC Blackout), you may benefit from going way above the recommended dwell time range.

The ideal zone for dwell time for the 5.56 NATO cartridge is 0.15-0.18 milliseconds, though some say it can go as low as 0.12 milliseconds and as high as 0.20 milliseconds. We can buy that, so we use 0.12-0.20ms as our target zone.

Once you get down below 0.12 milliseconds, even the most efficient gas systems will be under-gassed and will start to experience malfunction. Very few systems will function with a dwell time under 0.08 milliseconds.

When you get much above 0.20 milliseconds, most systems will be over-gassed.

Pistol Length Dwell

Please note that pistol length gas systems are rarely ideal (except for low pressure loads, like sub-sonic .300 AAC Blackout). The extreme port pressure confounds the principles of dwell time and it creates its own set of issues. If you choose a pistol length gas system, it will likely be hard to tune and you will likely experience excessive recoil, wear, and fouling, even if it cycles acceptably.

Rifle+2 Length Dwell

Please note that we do not recommend the Rifle+2 gas system length for the small frame AR. While the dwell time is technically acceptable, the port pressure at this location is very low for small frame cartridges. This low port pressure may cause under-gassed malfunctions (e.g. failure-to-feed, short cycling).

Suppressor Use

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. This is the reason the inside of suppressed guns will foul up much faster.

A dwell time heatmap, like the one below, can help us find the best combination of gas system length and barrel length. This heatmap is based on calculated dwell times derived from muzzle velocity data generated by RifleShooter.com for the Federal XM193 cartridge (note that this data is only accurate for this cartridge, but is typical of similar cartridges).

Please note that this evaluation assumes a standard setup, including:

An appropriately sized gas port for the stated system length
A standard mass bolt carrier
An appropriate buffer mass and buffer spring strength
An unsuppressed firearm

The following graph shows the data a different way. By plotting the calculated dwell time against the associated barrel length, we can estimate a continuous plot.

Using our calculated dwell times, we can determine a regression equation (second order polynomial is sufficient), and from this, calculate the ideal barrel length using our target dwell time range. We have provided these values in the inset table. For example, the ideal barrel length for a mid-length gas system firing XM193 ammo is between 13.41” and 16.47”.

We can also use this chart to identify the best gas system length for a given barrel length. For example, if you want an 18″ barrel, the best gas length is rifle length. If you want a 16″ barrel, the best gas length is mid length.

Below you will find some additional dwell time charts for common small frame and large frame cartridges, based on muzzle velocity data from RifleShooter.com