Quick Tip: What Is AR-15 "Dwell Time"?
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- เผยแพร่เมื่อ 18 ก.ย. 2024
- 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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These series are very helpful. Even as a refresher for folks who already know most of it…or think they know it. 🙂
After Caleb works on your AR it comes home wearing a bowtie.
If it's formal. Bolo tie, if it's feeling casual
And slicked back hair.
Chad status
@@STEVEARABIA1 Question: Can you use oil-based hair pomades as lubricant for firearms? I don't expect you can use water based pomades for this, but what about oil based ones?
it will be sweet and funny if it does
Best and most informative channel for DIY gunners like myself, bar none! Thanks Caleb, and thanks Brownells!
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.
This isn't exactly the kind of hobby where you can simply go down the list and check all the boxes and as long as you follow the rules you will be ok, this is the kind of hobby where it is critical you understand exactly why you're doing something and how it effects the system as a whole.
I'm a retired machinist and have worked on my pistols and rifles my whole life. Every problem that can cause a misfeed of any kind can be grouped into one category, TIMING. It doesn't matter what you call it, the timing of the mechanics have changed. 99% of the time this change is due to contamination of the moving parts or DIRT. It doesn't matter if it is actual soil or powder residue. The minute that any friction point is increased that's where the change in the function of the firearm happens.
A good cleaning and lubrication and the removal of any interference will restore your firearm into its original working condition. It's just that simple.
Clean your weapon!!!
I can't think of Brownells now without think'n Nerdery!. Appreciate the info..
Very good, highly useful production! After 10 years of Army armorer’s instructions either going over my head, or just not clicking, it’s very nice to finally understand this. Great job, much appreciated!
This is one of those videos that I wish TH-cam offered a ❤️ button alongside the thumb up button for. I absolutely love these technical vids because they get down to the nitty-gritty details of what goes on.
As a follow-up to this, maybe a discussion on how adjustable gas blocks work and how they can help or hurt how reliably the gun functions.
Thanks for the morning upload. I am learning gun technicalities on the toilet seat.
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.
PV = nRT ;)
@@jthrelf another engineer.
Learned something new today. Thanks Caleb.
Great explanation. Well done!
i believes these guys more than everybody else. this is way i subscribed
Thank u for this video. I was dying to make my 16in 556 into a “dissipator” or even use an intermediate gas length. But now i realize that a mid length gas system is the perfect option with an AGB. No point to risk the dwell time being too low with an intermediate or rifle length gas system on a 16in.
Thanks
Nice explanation
Thanks for taking the time to make these. I always leave informed 👍
Such a thorough examination. Thank you Brownells for these educational videos.
I picked up a Sig Sauer m400. Like the original not the tread or the snake bite. Anyway.. very first AR-15 I've ever owned. I'm 42 years old. Took long enough. I would like to say thank you to everybody at this channel. I've learned a lot from you folks and I really appreciate that. When I got my AR-15 like I knew how to load it I knew how to chamber around payment and fire it. I didn't even know how to clean it properly and I mean I've owned guns my entire life. Just never an AR-15 and then we know and AR-15s parts and whatnot move a little bit different than shotguns, pistols, bolt action rifles etc. Again.. thank you very very much. There is such a wealth of wonderful and very accurate and useful information on this channel. As always stay safe, keep carrying and God bless 🙏🇺🇸🏴
👍. Thanks again !
Great video as always thank you for all you do!
When shooting the 6.5 Grendel with a 22” or 24” barrel you can get what is called “belted” cases when the cases are being extracted while the pressure is too high. The case gets ballooned out and looks like a belt on a magnum cartridge. Proper powder selection will prevent this from happening. This is due to excessive dwell time on gas guns and fast powders.
We need more of this that’s awesome thank you
Thanks for this video! It was very informative as always. You’ve helped me understand how these firearms work. I had never heard this term so I learned something new.
This explains why Pistol and Carbine length Gas systems does much better with cycling Steel Cased ammo.
My rifle length gas system has no trouble with steel cases. So I guess I am lucky.
Love videos like this wise you guys made lenghtly videos these are wayyyy took short
Always interesting and useful information, thank you.
Caleb is a treasure
Excellent explanation and clarification. I've heard other channels get this wrong and say midlength has "more" dwell time.
Caleb made a video without a cup of coffee? who is this imposter...?
Nerdy is good. Stay that way. WE like it. Thank you.
A video on what upgrades contribute the most to accuacy in order of % of improvement. In other words what contribtutes the most to least so what would get a person the greatest increase in accuracy. Barrel, free float, ammo, BCG, trigger, reciever group?
If your build is solid and the barrel is decent, the ammo has the largest impact on accuracy apart from the shooter, which is the single largest variable. Outside of ammo, the next thing is whether the receiver and barrel extension are true to each other and the quality of the barrel. Apart from that, free floating the barrel will generally provide tighter groups, not not always. You can have a better barrel in a non free floated confit that will group better that a free floated config. If you’re hand loading you can also tune the load to match your specific rifles harmonics too.
As far as parts upgrades, AR accuracy is 90% about the barrel, followed by the trigger, then free float. Free float is almost irrelevant if the barrel won’t group and the trigger isn’t consistent. ARs are not bolt guns, and certainly not benchrest guns. Anything you do beyond barrel, trigger, and free float is searching for minuscule improvement, which may or may not be noticeable depending on your ability- and most likely shooting from a bench.
How to shoot accurately enough to tell the difference is a lifelong pursuit.
Accurate ammo is a given. A national match rifle isn’t going to group with crap ammo.
Great video 😀 my husband and I will be making a purchase soon. God bless all of you.
Brownells 1939-2024 RIP...We will miss you !!! Corn by one's onw bushel.
Excellent information. Thank you.
We love the nerdy! we need more of this, especially for the ones that love to know the "science" of weapons. Thanks Bro! Keep the "Nerdy".
Well put. Not redundant just drove the pont home
Dwell time = the time gas is acting on the system. The time gas is acting on the system = how long it takes the bullet to go from the gas port to the muzzle. So, dwell time = how long it takes the bullet to go from the gas port to the muzzle.
Edit: In a DI AR. In a piston driven action, dwell time will be influenced by the gas release porting of that system.
I appreciate the redundancy! Thank you for sharing!
As my PawPaw used to say "Knowing and not knowing too , I'd really hate to say either way" 👍🏼
Great information, just the kind of 'quick and dirty' info we need (been thinking about changing our my carbine length barrel for a 20 inch with rifle length gas port (I understood the gas tube should change), I'd like to more about the impact (of not change buffer, spring, or bcg...) Thx.
A case of splitting hairs. While your definition is correct and I see it..... I think the general thought a lot of people have is, dwell is the time of positive pressure enacting on the BCG. SO, length from gas port to muzzle is the time you have to make positive pressure. the shorter the distance between gas port to muzzle the less space you have to make that positive gas pressure. So fixing dwell time can get a little "deep end" with diameter of the gas port and adjustable gas blocks, length of gas tubes (seen them wrapped around a barrel before), weight of BCG and buffers to get proper dwell time for proper clearing and ejection.
exactly!
And powder burn rate. Which is why the definition is pressure acts on the action, that may not be until the bullet exits the bore.
This is exactly what folks need to hear.
Looking forward to more videos like this 👍🏻
Great, great explanation
Good Video/Info.
This is really good information. Reminds me of ignition coils, blow-by, or back-pressure in an engine. So then using a dB lowering suppressor will increase dwell time? Can y'all do a follow-up video with common misdiagnosis and proper diagnosis of dwell time related malfunctions and causes?
A short answer for your suppressor question would be yes, a suppressor can increase dwell time, due to the pressure captured within the suppressor body, but not as much as a longer barrel produces as the gas pressure has more room for expansion within the suppressor body and baffles. As for the issues caused by dwell time, this gets a bit complex as there are three other factors at play within a semi-automatic gas cycled rifle. Along with dwell time/gasblock fitment and alignment you must factor in spring strength, buffer weight, and power of the ammunition you are using. The most I can say is ensure your gasblock is properly aligned using either a tool or the spaghetti trick and then if issues occur you at least know that you have ruled out one factor. Hope this helps!
@@Zel_eo Dude, thank you so much for taking the time out of your day to give me some helpful insight and sharing your knowledge with me. You're a good person. Have a gooder.
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
No, more dwell time is not always better, longer gas systems produce a shorter dwell time.
Thank you Sir.
Finally! Somebody who defines dwelltime the same way I define dwelltime.
So Kaleb said that dwell time wasn't the amount of time the bullet spent in the barrel after reaching the port, but the amount of time that the port gasses acted upon the block? Yet he didn't explain why the two would be different?
Because you are always trying to solve for a constant. Dwell time is always = the lengths of the barrel after the gas port, but the length of the barrel is a changing variable so we use the constant of “Dwell time”. That probably makes even less sense than when he explained it but that’s just how physics do. Force is always = to mass x acceleration, and even if the mass or acceleration (barrel length after the gas port) change the Force will always be the product of those variables.
That distinction wasn't explained clearly enough for me either.
@@Jonathan906 me 3. The instant the bullet leaves the barrel, the pressure in the gas system drops to zero. I guess it takes a fraction of a second for the gas pressure inside the BCG to dissipate? No, that’s wrong. Gas pressure in the BCG should drop the instant the BCG moves back far enough for the carrier key to disengage from the gas tube. Also there are the vent holes in the BCG… Caleb has me thinking for sure. And scratching my head.
Thanks Caleb for the detailed information 👍
That LWRC BCG 👌
Many people confuse Dwell Time with Lock Time.
Ok since there is an interrelationship between dwell time, barrel length, carrier weight/dimensions and recoil buffers and springs, is there a chart that shows this relationship?
There is a table with four entries on it. It lists how big a hole to drill on the barrel so enough gas is tapped to provide a sufficient pressure pulse for each barrel length to cycle the action.
Carbine length have a smaller holes than rifle length barrels because the pressure tapped closer to the breech is higher.
In the AR-17 and AK-47 dwell time is determined by how long the bayonet handle you stick on the rifle is. Reciprocating mass and spring rates only matter when talking about full auto rates of fire.
Dwell time is the amount of time it takes for your bolt carrier group to begin the movement of extraction of a spent cartridge.
In other words, the time the BCG 'dwells' in place after a cartridge is discharged.
Dwell time is measured in fractions of a second.
This is affected by the length of you gas system, the powder load in your ammo, the back pressure of a suppressor, and the weight of your buffer and bolt carrier.
All of those factors can be tuned to adjust accuracy, extraction pattern, felt recoil, and wear and tear on your rifle. It can mean the difference between a smoothly and comfortably operating rifle, and regular malfunctions, stuck cartridges, and premature parts failure.
You know after you fired your first magazine from a fresh build and you sort of have that glowing warm fuzzy feeling and you want a cigarette!…..Dwell time!
Now I feel all geeked out. Dang I need a shower!
Great video. Very informative, thank you.
Sweet ported barrel
Caleb's voice is so soothing it's kind of like Bob Ross but with guns instead
The description that you just gave that determines dwell time, is the exact time that the bullet passes the gas block, until it exits the barrel tip. Alot of people refer to it as cork time. The amount of time that the gases interact upon the cycling of the gun dies not determine the amount of pressure applied at all times, no, but it does describe the time for which gases are applied to the gas tube, which is the anoint of time that it takes the bullet to pass the gas block, and exit the barrel. You didn't ask what the pressures were along that time, you only asked what dwell time was.
Then you went into the specifics of gas pressures, buffers, and spring tentions, which all affect the performance of the gun, but your original question was what is dwell time. The dwell time is the amount of time that the gases enter the gas block, which us when the bullet passes the gas block, until the bullet exists the end of the barrel, period. Do thise pressures vary at the instant tge bullet is fired until it exits the end of the barrel, yes, but the amount of time varies little, which if you wanted to get extremely technical, those times can be affected by the exact amount if powder, bullet weight, temperature, and other factors, but dwell time is the amount if time on average that the bullet spends between the gas block and the end of the barrel, it's that simple! It is also called cork time, and in my opinion is a good descript to what is happening because it corks the barrel and applies the gas to the gasblock, the gas tube, and the bolt carrier and bolt.
Very well explained
I'm wondering if a slow-motion camera could capture the dwell time it takes to perform such a slick hair.
Great job!!!!!!
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.
Yes, Think of it this way, the closer the gas block is to the muzzle, the less time the gas will be pressurized in the firearm.
@@CalebSavant
Copy - Thanks
Love this stuff
The comments will be all over the place on this one 🤷♂️👍😁
Nerdy is good.
Dapper AF
🤠🤝
Awesome video per usual!
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.
FYI... motors used to have point in the distributor and yes I'm old...just wanted to get that out of the way...😏😆😆
Just what I was thinking.
@@georgesakellaropoulos8162 😁...you will be my age someday 😆😆😆
@@ETHRON1 I might already be.
Calab, wish you had a zoom class to build own AR & AK with Q&A
Gas related issues frequently come up.
Often shortly after a meal.
Mostly on taco Tuesday.
You said that dwell time is “the amount of time that the gas is acting on the system”. So, once the gas unlocks the bolt and the bolt carrier moves back far enough to get the gas key off of the gas tube, the gas is no longer acting on the system. That tells me that a pistol length system has the shortest dwell time, not the longest. Also, the carbine gas system would have a shorter dwell time than a mid-length or a rifle length gas system. I agree that a shorter gas system has a higher pressure than a longer one but, I think that dwell time is directly proportional to the gas system length. Why am I wrong?
Pressure curve drops as it occupies space in barrel and gas tube. It’s also dependent on powder burn rate. That’s my take on it!👊👊👍.
@@Eyeontheogive That only further illustrates what Michael Peterson stated.
You could have explained it using physics. Pressure equals force over area. P=F/a. When the surface area of the barrel increases with length, the force of the gas in the barrel is distributed over a larger area which results in a lower Pressure.
If knowing the inner workings of a gun make me nerdy, then I want to be Urkel
The relationship between barrel length, gas system, and the 5.56 cartridge would have been appropriate to talk about here. For example , the ideal setup is a 20" barrel with a rifle length gas and buffer . Everything else is a fix , or work around for a shorter gun.
Ok ok let me get this straight
Dwell time is the amount of time gas is acting on the system and it is not the amount of time it takes the bullet to go from gas block to leaving the barrel.
My problem here is that the gas starts to act on the system as soon as the bullet passes the gas block and it stops acting on the system when the bullet leaves the barrel. *Passing the gas block = gas starts to act on the system.
*exiting the barrel = the gas stops acting on the system
Therefore dwell time does equal the amount of time it takes to go from gas block to exiting the barrel. It may not be the technical definition but it does infact give you the correct answer. Once you define the points where the gas starts and stops acting on the system both definitions are correct.
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?
@Alex Gessner Sounds reasonable.
Dwell time is very correlated to gas port to barrel crown. Shorter distances:less dwell time
A question I’ve always had is how much length of barrel past the gas port is required for the action to cycle. For instance, if you cut the barrel length shorter, say for example to SBR a rifle, how short can you cut it past the gas block? Not just for an AR15 but for piston guns as well? To be even more specific, if I wanted to SBR a Mini-14, how short past the gas block can I go and still be functionally reliable?
You mentioned ejection angle... how can you use that to diagnose gas problems?
I go into detail on our series we did, check it out on the website, unfortunately I cannot link it for the same reasons it was not allowed on TH-cam.
Use your Right hand when you suggest we smash the like button, YT moved it (again)
Is it me the only one that misunderstood? He did say Dwell time is the time the gas takes part in your system (not the time it takes the bullet travel from the barrel gas hole to exit the barrel). check it out 5:12. Please advise Brownells, Inc.
D.I. guns are history!
Old guys can think of this as ignition timing
Powder burn rate can affect dwell time. Too fast a powder and you can obtain max pressures and velocity but the gas at the port of a rifle length system my not ne enough to cycle.
OBTW, lets get an update on the rude comments (I find those video hilarious...)
Hey Newby here. So, how does the port (gas) size and position and buffer weight effect calculating a build to accommodate smooth reliable operation and longevity. And how to calculate that. I.E. small port near chamber vs large port near muzzle. Thanks
I can relate, I have a gas related malfunction after eating Taco Bell.
What about when using adjustable gas blocks ? and larger gas ports.
Won't affect dwell time, but will affect pressure, which will affect bolt velocity, etc. There are so many factors to consider in a gas-operated system, talking about one by itself is kind of pointless.
Most barrel makers have a set gas port size for a given length. Rifle length will have a larger port than mid and so on down the line. Other things like barrel diameter, twist rate, etc., all have an effect. I believe there are industry size range standards for these port sizes, but each maker will have their own specific depending on barrel design.
So just to be clear, a carbine length gas system has more gas then a mid length ? I am fairly new to this and I am having trouble with my current build.
12.5inch barrel with a carbine length with a 3oz buffer tube and I am short stroking every couple rounds. I thought I was because I didn’t have enough gas and I am about to order a mid length gas system to fox that. But from my understanding the carbine length should be producing more gas ?
Thanks
This is confusing. If there are higher pressures with shorter gas systems, then why are there higher bullet velocities with longer barrels? As the bullet travels down a barrel, the space/gap between the empty case and the back of the traveling bullet increases, so does the volume of that space. Increasing volume will drop pressure unless the burning gas/powder can add pressure at a rate faster than that is being lost from increased barrel volume. The longer barrels mean more powder gets burned and not lost out the end of the barrel right? And I thought that with a Carbine length gas system, the gas has a shorter distance to travel and act on the BCG, in contrast a rifle length has a longer distance to travel back to the BCG giving it a longer dwell time. please clarify
Which is better an adjustable gas block or adjustable buffer weight, can a rifle be more finely tuned using both?
I have a gas problem, I need advice.
I thought dwell time was the amount of time it takes a fart to clear a room
Always was taught it had to do with distance between end of the barrel to the port in the gas system….thought that this is what truly affected reliability. This is based on gas pressure exerted from rear of bullet into the gas port after firing…that time where the bullet passed the gas port and allowed the gas to interact with the port and the bcg. Did I have this wrong?? Caleb??
No, that's correct. In a direct impingement AR, the distance of the gas port to the muzzle determines dwell time. A pistol length gas port 6" from the muzzle will have the same dwell time (but higher pressure) as a rifle length gas port 6" from the muzzle.
The reason it might not be considered correct in all circumstances for all firearms is because in, for example, a piston operated system, the gas relief porting of the piston also determines dwell time. So his definition is correct for all gas operated systems, but the "port to muzzle" definition is correct for all direct impingement ARs.
Thanks bud. This is a 30 minute video if you ask me.
Wouldn't a projectile take longer to exit a longer barrel? Or Did he just make a mistake?