Brownells Gun Tech™ Caleb Savant dwells on a topic that’s important when you’re diagnosing a cycling malfunction on an AR-15 or choosing the gas system for a new AR-15 build: dwell time. “Dwell time” is the amount of time the combustion gases from the fired cartridge act on your AR-15’s operating system. When you fire a round, the bullet is propelled down the bore by hot gas. After it passes the gas port, some of the gas escapes through the gas block into the gas tube, which takes that gas back to operate the bolt-carrier and cycle the action. If you web search “AR-15 dwell time,” you’ll get a lot of hits on a different answer, and Caleb explains why it’s not entirely accurate.
You’d need ultra-high-speed laboratory instruments to measure actual dwell time, so a simple indicator of dwell time is GAS SYSTEM LENGTH (the distance from the chamber to the gas block). Different gas system lengths provide different dwell times. Shortest to longest, the standard AR-15 gas system lengths are Pistol (about 4.5″), Carbine (~7″), Mid-Length (~9″), and Rifle (~12″). For example, if you have two 16″ barrels, one with a Carbine-Length gas system and the other with a Mid-Length, which one has the longer dwell time? Write down your answer and see if it matches Caleb’s answer! Also, the closer the gas port is to the chamber, the higher the pressure of the gas that hits the gas tube. The farther the bullet travels down the barrel, the less gas pressure behind it. So a Pistol-Length gas system generates substantially MORE pressure than the much longer Rifle-Length system.
The Big Takeaway: DWELL TIME is the total amount of time the GAS pressure from a fired cartridge acts on an AR-15’s operating system.
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Thanks Caleb for the detailed information 👍
2 to 3 minutes. 5 if I'm lucky.
Completely overlooked the point of why dwell time is important! For reliable case extraction and ejection! More dwell time is better! Longer gas systems produce more dwell time. Gives the malleable brass casing more time to contract after initial expansion from firing
These series are very helpful. Even as a refresher for folks who already know most of it…or think they know it. 🙂
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I've heard of looped gas tubes for short barrels that are supposed to simulate carbine gas tubes. Do they really work or offer any real advantages?
Very well explained
Love the video!! So much that goes into the proper operation of an AR. HEY, CALEB! Now that you mentioned the different gas systems (pistol, carbine, mid-length and rifle), if I were to switch out my upper assembly from a 5.56 to, let's say, a 300 blackout, do I need to be concerned with the gas system? Should the gas system on the 300 blackout be the same as the 5.56? Do I need to also consider the buffer tube? Fascinating! Anybody else have ideas? Thanks, y'all! Keep 'em comin', Caleb!
Best and most informative channel for DIY gunners like myself, bar none! Thanks Caleb, and thanks Brownells!
Thanks caleb
This explains why Pistol and Carbine length Gas systems does much better with cycling Steel Cased ammo.
Always interesting and useful information, thank you.
The pressure behind the bullet decreases because the bore volume increases as the bullet moves forward in the bore. When first fired the gas volume is essentially the cartridge case volume. As the bullet moves forward the volume behind it increases rapidly decreasing the pressure faster than the burning powder can increase it.
Verbal gymnastics. The amount of time the gas pressure acts on the action is equal to the time it takes the bullet to travel from the gas port to the muzzle. Bore pressure drops to zero (or 1 atmosphere) when the bullet exits the barrel muzzle .
Would you address the same "dwell" time for piston operated rifles…?
We need more of this that’s awesome thank you
Define "acting on your firearm system". You're splitting hairs. As long as there is blast pressure moving down the gas tube then it is acting on the action. Since that pressure only exists during the time the bullet goes from gas port to barrel end, that is the only OBSERVABLE way to measure dwell time. Your failure to define the "acting" term only confuses things and leaves the viewer still wondering what dwell time is.
Sweet ported barrel
Good beginning, but how do you affect the dwell time and work with tuning the system. I know that changing cycling includes the buffer tube and spring, but can the buffer tube change things? What about the weight of the GCG? I have an inkling of an idea how these things affect cycling, but that's just it, an inkling. thanks,
First you said dwell time wasn't what everyone said and then you turn around and defined it as such.
Nerdy is good.
Did I understand you – A carbine-length gas-block has more dwell that mid-length of rifle-length? Inversely proportionate, i'e., the shorter the length from the chamber, the longer the dwell.
He said the definition was wrong then explained it's the length of time the gas has on the system depending on the distance the bullet has to travel past the gas port! So the definition is right.
Learned something new today. Thanks Caleb.
It is good to have this technical definition of the term, and even noting that the common use variant is also "correct" for visual explanation. This seems to provide clarity for folks who want the knowledge expansion and it's appreciated. Just one piece of the puzzle, folks. No need to get all flustered over it.
Going on the basis of what you are saying then why do dissipator like systems not work well? They are pro-ported to have ‘bad’ dwell time based on the lack of gas coming back because the barrel is too short.
I'm wondering if a slow-motion camera could capture the dwell time it takes to perform such a slick hair.
Great explanation! Thank you.
I love all the conflicting terms and nomenclature in the gun world. People freak out when someone mixes up clip vs magazine or utters "assault weapon," as if those are carved in stone terms yet what should be a very clean cut, almost scientific term is clearly a contemptuous topic.
I for one thought 'dwell' was the delay from the bolt moving caused by the longer gas system. Longer gas tube = further for the gas to go = delayed and slower moving bolt. Guess I was wrong what was being described by the term, but I think functionally I am still correct though.
Thanks
Unfortunately this is a very poor explanation of the topic of gas gun cycling. Failure to identify chamber dwell time and differentate it from cycle/ op system dwell time kills this whole discussion.
The timing of the unlocking of the bolt, the size of the gas port and the distance from the port to muzzle all effect the cycle of operation. Unfortunately it gets even worse when you start including variables in loads, ie light 55gr vs heavy 77gr 556 loads. Most shooters can get away with mass regulation if they bought a properly gassed barrel, ie change the buffer until you get proper ejection. If you start changing carrier weights and messing with gas regulation you are going to get into the weeds real quick.
Caleb made a video without a cup of coffee? who is this imposter…?
I always heard that rifle length gas systems seem to erode at the gas port at a higher rate than carbine/mid. Seems counter intuitive but maybe I am completely wrong
If the definition is the "amount of time the gas is acting …", then does one need to account for the time it takes for the gas tube to pressure to the point where some action is actually occurring? Similarly, is it possible for that pressure to drop before the bullet passes out the end of the barrel? If the bolt is slammed all the way back before the bullet exits, then is the dwell time considered to be over?
Well put. Not redundant just drove the pont home
It's not that different from the "Dwell time" with a set of points on a motor…same basic principle. Good to know "C"…added this to my gun tech Playlist.
THANKS
Re-name the title "gunsmithing for idiot's "