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Let's talk about different gas lengths with adjustable gas blocks


Maxamundo

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This has been something I've been trying to wrap my head around for a long time as I'm trying to build the softest shooting AR-15 possible.

There is so much incomplete info on what it means to have midlength vs rifle gas and etc etc and which is softer shooting and all that. I'll ask my question and then talk through my thought process so far, and hopefully someone can help me get a more complete understanding of the platform.

Basic question: 

  • Assuming all other components of the AR are kept constant, and the only thing we change is the gas port location on the barrel, why should there be any difference in felt recoil at all, given that we adjust the gas block to just lock the bolt open at every gas port location?

I've made an assumption here, and that is that even when running an adjustable gas block, there WILL be a difference in felt recoil depending on gas port location. I feel that this is a pretty safe assumption to make since I've seen an overwhelming amount of recommendations, specifically from the 3-gun community, for choosing an 18" barrel with rifle gas and an adjustable gas block to get the softest shooting rifle. Not only that, but I've NEVER heard anyone claim that mid length or carbine gas feels exactly the same as rifle given you adjust your gas block accordingly.

What I know so far: 

  • First, lengthening the gas system essentially does only one thing at the basic level - changes the pressure at the gas port as the bullet passes. Approximate pressures are shown on this graph.
  • Second, the BCG will not start rearward movement until pressure at the carrier reaches a certain value (determined by the force exerted on the carrier by the buffer spring).
  • Third, adjusting the gas port hole size (through an adjustable gas block) changes the rate at which pressure builds up at the carrier.

What I don't know so far:

  • How does the time required for pressure building up at the carrier compare to the dwell time? Are they on a similar scale, i.e. 100-300µs, or does pressure at the gas key build nearly instantaneously after the bullet passes the gas port, or does it not reach it's highest pressure until well after the bullet has left the barrel? Again, here is another graph that will help.
  • Why would a more open rifle length gas port cause the carrier to accelerate any differently than a more closed mid or carbine length gas port, given that we close the gas holes enough so that the time it takes for pressure to build up at the carrier is the same as with the rifle length system?

Intuitively I feel that the lower pressure at the rifle length gas port will accelerate the carrier more slowly/smoothly than the higher pressures of the mid or carbine, even if total rearward kinetic energy of the BCG is the same for each system. But I still feel like I'm missing a lot of what is going on inside the gas tube, and specifically I hope someone can shed insight onto what a pressure graph at the carrier might look like with each gas system.

 

Edited by Maxamundo
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Max, I've been curious about this myself.  I'm not sure I agree with your assumptions (what you know), except that the final BCG velocity is the same since it's stopping rearward travel at the same point (just locking back).

The pressure at the port is higher with a shorter system, but the actual operating pressure is determined by the gas port size.  A restriction like that acts like a resistor (the port) in a circuit, dropping voltage (pressure).

 

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Hmm...  I thought those 3 statements I made were pretty solid, can you go into more detail on why you don't agree with them? Really curious. 

I disagree with you that the final BCG velocity is the same simply because it stops at the same spot. By final velocity do you mean initial velocity (immediately after unlocking)? Otherwise the velocity would be zero at the compressed part of the cycle. It's tricky because I'm not sure how the time force is applied to the bolt carrier by the gas system changes depending on gas system length. For example is a carbine length gas more of a "hit"  on the carrier where the rifle length is a "shove"? Or maybe vice versa because the dwell time determines how long pressure is high in the carrier?

What I think is true is that the final work done on the spring is the same, that is it has the same potential energy at the time when it's fully compressed and the BCG has velocity zero. 

Edited by Maxamundo
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Don't know nothing about no pressure balancing by choking down the port, but I do know if you have a smaller cup it will fill up much faster than a bigger/taller cup no matter how much you choke the water back. ;)

You actually answered your own question. Port pressure is  higher in short systems. Doesn't  matter how much you restrict it, 5000 psi more in short tube is going to make the carrier react faster, than less pressure in a longer tube. Reacts faster = sharper feel to recoil. Plus the short tube fills up much faster with the higher pressure. Also this isn't a pure gas it performs more like a plasma and in so being isn't as compressable.

Edited by kurtm
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Kurt, what I'm struggling with is understanding what exactly the gas block is doing then? If you assume that the gas tube has 5000 more psi on a carbine system even if we constrict the gas port, then why did we constrict the gas port in the first place? Timing has to play an important role here, I'm trying to understand how it fits in. 

Just speculating, but let's assume that the amount of time it takes for pressure to build up in the carrier is orders of magnitude smaller than the time it takes the carrier to move enough for the gas rings to pass the bleed off holes on the side of the carrier. This means that even with a gas block closed off to 99% or so on a carbine length gas system, you still reach a maximum pressure much higher then you will on a rifle system. 

Working under this assumption, this would mean that in practice, putting an adjustable gas block on a carbine length gas system might only get you 10% of your "maximum recoil reduction" , where putting an adjustable gas block on a rifle system might let you get 50% of your "max reduction". This is simply because in practice, it would be impossible to turn off the carbine gas precisely enough to get the exact constriction that would be ideal (I'm talking like theoretically maybe 1/100th of a turn of the gas adjustment screw from completely off). 

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Don't fully understand it but I'll throw this out there.

On a carbine gas system, with the adj gas block set to a minimal setting to function the action, relative to the longer gas system it will extract at a higher chamber pressure.  Doing that requires more energy into the carrier in total, though it should also use up the extra energy.

So maybe on a carbine gun it is using 75% of gas energy to extract and 25% to cycle action, while a longer gas system is using 50% of on extract and 50% on cycling.  Which would make the carbine gun have more variance in its cycling, requiring it to be set with extra average gas to ensure 100% function?

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1 hour ago, Maxamundo said:

Hmm...  I thought those 3 statements I made were pretty solid, can you go into more detail on why you don't agree with them? Really curious. 

I disagree with you that the final BCG velocity is the same simply because it stops at the same spot. By final velocity do you mean initial velocity (immediately after unlocking)? Otherwise the velocity would be zero at the compressed part of the cycle. It's tricky because I'm not sure how the time force is applied to the bolt carrier by the gas system changes depending on gas system length. For example is a carbine length gas more of a "hit"  on the carrier where the rifle length is a "shove"? Or maybe vice versa because the dwell time determines how long pressure is high in the carrier?

What I think is true is that the final work done on the spring is the same, that is it has the same potential energy at the time when it's fully compressed and the BCG has velocity zero. 

The gas is only doing work on the BCG when the key and gas tube are mated up.  Once they separate, it's all carrier momentum after that.  If they're both tuned to just barely lock back on empty, then the starting velocity has to be the same.

You'll have to press the "I believe" button on this part: a restriction in the gas system works like a resistor in an electrical circuit.  That principle is a modeling tool (all models are inaccurate, some are useful) I remember from college.  

In any event, let's say we have two gas ports; A & B, port A is at a location in the bore that the pressure is 10k psi, and port B is 8k psi.  I could size the port at location A to get a gas tube pressure (what I referred to as operating pressure before) of 3k psi.  I could also size the port at location B to get a gas tube pressure of 3k psi, it would be bigger than the port at A.  But if port A is further from the muzzle, it'll take longer for that port to bleed down to atmospheric after the bullet is gone, and it'll get pressurized earlier.  So I would probably reduce the operating pressure below 3k using port A to get less force but longer duration to equal the higher force and shorter duration of port B.  

All that said, if there is less recoil on a rifle gas system and the final BCG velocity is the same, the only other factor is momentum transferred to the rifle while the bolt is still locked up.  Maybe a short high force gas pulse, right before the bullet leaves the barrel pushes the bolt forward (while pushing the carrier rear) keeps the rifle "stationary" and allows the momentum to transfer cleanly with the carrier without excess gas driving the carrier long after the bullet is gone.  

It's far from a clean thought, but I think it has a lot to do with distance the port is from the muzzle.

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1 hour ago, Maxamundo said:

Kurt, what I'm struggling with is understanding what exactly the gas block is doing then? If you assume that the gas tube has 5000 more psi on a carbine system even if we constrict the gas port, then why did we constrict the gas port in the first place? Timing has to play an important role here, I'm trying to understand how it fits in. 

Just speculating, but let's assume that the amount of time it takes for pressure to build up in the carrier is orders of magnitude smaller than the time it takes the carrier to move enough for the gas rings to pass the bleed off holes on the side of the carrier. This means that even with a gas block closed off to 99% or so on a carbine length gas system, you still reach a maximum pressure much higher then you will on a rifle system. 

Working under this assumption, this would mean that in practice, putting an adjustable gas block on a carbine length gas system might only get you 10% of your "maximum recoil reduction" , where putting an adjustable gas block on a rifle system might let you get 50% of your "max reduction". This is simply because in practice, it would be impossible to turn off the carbine gas precisely enough to get the exact constriction that would be ideal (I'm talking like theoretically maybe 1/100th of a turn of the gas adjustment screw from completely off). 

The adjustable block is restricting the total amount of gas not reducing the pressure.

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37 minutes ago, busdriver02 said:

It's far from a clean thought, but I think it has a lot to do with distance the port is from the muzzle.

This is why I want to run a 16" barrel with rifle gas as an experiment. It's probably going to involve getting a 16.5" rifle gas barrel and cutting it down by a half inch. 

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Max, you need to spend a bit more time on gas laws and how plasma flow. Pressure is pressure. Think about a garden hose. It is flowing at around 40 psi. When you put your thumb over the end the water squirts farther. Did you increase the pressure in the hose? If you answer yes you need to spend a lot more time studying! There is no way putting your thumb over the end changes the pressure, think about the 12" water main that supplies the water to your house.....do you really think that by putting you thumb over the end of your hose it increases the pressure of that?? What you have done is changed the velocity that the water flows at, not the pressure.....now think about that!!!! Port pressure!!! Now don't  think, just drink! ;)

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Sorry, forgot to add:  The force on the BCG from the spring is essentially inconsequential when you compare it to the forces involved.  The carrier starts moving backwards essentially right away, it's the bolt that's delayed since it's being pushed the opposite direction as the carrier.

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23 hours ago, kurtm said:

Max, you need to spend a bit more time on gas laws and how plasma flow. Pressure is pressure. Think about a garden hose. It is flowing at around 40 psi. When you put your thumb over the end the water squirts farther. Did you increase the pressure in the hose? If you answer yes you need to spend a lot more time studying! There is no way putting your thumb over the end changes the pressure, think about the 12" water main that supplies the water to your house.....do you really think that by putting you thumb over the end of your hose it increases the pressure of that?? What you have done is changed the velocity that the water flows at, not the pressure.....now think about that!!!! Port pressure!!! Now don't  think, just drink! ;)

So, in the case of a carbine gas system, if the aim is to reduce pressure at the bolt, would not a bleed off gas block like the Superlative Arms or the bleed off adjustable bolt carrier like those from 2A or Bootleg be a better mouse trap?

Mick

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Lot of good information being shared in this thread.  One big thing that you have to remember is that a carbine length system requires a larger gas port and more pressure because it's starting the extraction process earlier in the firing cycle.  It begins trying to pull the case out of the chamber while the case is still expanded, which requires more force and usually beefed up extractor springs or buffer inserts.  This all gets more complicated when you start reducing the length of the barrel after the port "dwell time."

 

By going with a longer gas system, you allow the fired case to expand and contract fully before the extraction process.  There are a ton of other variables beyond that, that can effect felt recoil.  

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this thread lost me, to some extent...  i dropped physics in HS and in college.   

my thinking  since you are dampening/tuning with adjustable gas block  the question is a wash.... since the adjustable gas system will dampen these forces.   sure the question of pressure is valid but i don't think the shooter can feel the difference on  input of gas to the bolt which is transferred to the rearward motion ob bolt/BCG

i believe that the whole mid length gas, vs rifle length gas  conversation only applies to NON adjustable gas systems.   because adjustable systems dampen gas forces, and decelerates the rearward movement of the BCG.   F=M A      newtons  second rule    the A (acceleration)  is equal   mid or rifle length   which is  the minimum force to lock bolt back.       

 by using light weight BCG and buffer system (reciprocating mass)   decreasing M in the formula  will provide you more results in decreasing muzzle flip/ perceived recoil

often overlooked.... one should consider tuning the forward action of the BCG  which     contributes to muzzle dip

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6 minutes ago, biglou13 said:

 

i believe that the whole mid length gas, vs rifle length gas  conversation only applies to NON adjustable gas systems.   

Sorry, but no. 

I don't have any math or formulas to prove you wrong but I do have experience with mid/intermediate/rifle length systems with similar pertinent parts and the same ammo. There is a difference. Its noticeable if one tests parts and does more shooting than debating on forums.

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A few observations from a guy with a 14.5" midlength gas "combat gun", several carbine gas 16" "combat" guns, a 16" carbine gas 3 gun, a 16.5" rifle gas 3 gun, 2 x 18" midlength gas 3 guns ( one with an A5 buffer) and several 20" AR service rifles.  All the 3 gun rifles have adjustable gas systems and Low mass BCGs.

- While subjective, I do believe that the rifle gas system will produce a perceived lesser recoil; I think that this is due to the lower rearward velocity of the BCG.  Or to put it another way, the longer gas gives more of a shove vs a sharp push.  I should think that energy required to return a carbine system vs rifle system would be about the same but the rifle system takes longer to operate so if feels less. 

- I believe that MOST rifle gas ports need to be bigger then the same barrel with a carbine gas system. 

- While my 14.5" and 18" guns are set up a little different, I think that the 14.5" midlength gas does produce less recoil and neither one of these guns have comps on them. 

- Rifle gas systems on a 16" barrel work just fine but the margin of operation is smaller then my 18" midlength guns.  My 18" mid guns will run a wider variety of ammunition from light 40 and 55's to 77's all set to a 77g min-operation gas setting. 

- an adjustable gas system does not do as much on a 20" rifle gas barrel as it does on a 16" carbine gas barrel, I can crank the gas way down on the 16" carbine gas barrel, not so much on the 20", this tells me that the 16" carbine gas guns are getting way more gas then they need to operate or are generally overgassed.  As such, these over-gassed carbine gas barrels benefit quite a bit from the adjustable gas system.

- I am confident that once you put aj adjustable gas block and low(er) mass bolt in an AR the gas port size is no longer critical as the adjustable gas block no becomes the limiting factor. 

 

 

 

 

  

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50 minutes ago, dauntedfuture said:

- I am confident that once you put aj adjustable gas block and low(er) mass bolt in an AR the gas port size is no longer critical as the adjustable gas block no becomes the limiting factor. 

This is true to a point, but only until you don't have enough gas.  Adjustable gas blocks are more restricted than a non-adjustable, even in their wide open setting.  Most manufacturers over size their gas ports for an extra margin of reliability because no two guns are exactly the same when they come off the line and not all ammo is created equally.  Take 300blk for instance, they typically are never over gassed in a non-suppressed situation.  I helped a guy set up his 300blk because he wanted his 8.5" SBR (pistol length gas tube) to run reliably with and without a suppressor.  Because Suppressors increase the back pressure enormously, he wanted to put an adjustable gas block on and be able to tune it to run at normal levels with the can attached.  The problems started after we installed the adjustable gas block.  Without the suppressor, the adj gas block was restrictive enough, that the rifle did not produce enough gas to cycle it reliably anymore (in the wide open setting).  So were left with a couple of options, start reducing mass in the buffer and carrier, or increase gas port size.  We opted in this case to go down in buffer weight and found reliability again.  Now attach the suppressor and watch the brass fly another 10 feet away, so we start cutting the gas at the adj block until we found the setting that allowed reliable lock back on the last round and the brass was landing at the same spot as the non suppressed setup.  He made a few notes in his book and was happy again. 

 

Working on various length and caliber short barreled AR's has taught me a lot more than 16", 18" and 20" variations over the years.  Even the manufacturers can't decide on a standard gas port size because of infinite variables like how many rifling's are in the bore, bore tightness, how deep the lands and groves are, the chamber dimensions, the overall finish on the chamber walls, and on and on.  If you take an 11.5" 5.56 barrel, with carbine length system, you can usually get away with a gas port that is (for the sake of numerous arguments) .065-.070.  There is sufficient dwell time after the port that there is a long enough pulse that sufficiently pushes the carrier back.  Because its a carbine length system, you are still trying to extract the fired case out of the chamber and have to overcome the extra friction of dragging the case against the walls of the chamber before it has had a chance to contract back down in size.  This requires beefed up extraction and you see a lot of extra wear and tear on all aspects of the brass, but 11.5" barrels can be made to run reliably with all sorts of ammo.  A 16" barrel with carbine length system would run with the same reliability at about .060-.065 on the gas port. 

 

Now shorten the barrel another inch to 10.5" or down to 10.3" and your problems will begin with finding the same reliability as the 11.5".  You have less dwell time, so even if the gas ports are the same size, you will not get the same volume of gas at the key.  Most manufacturers have learned that they have to open up the gas port to .075 or .080 to make them reliable.  The manufacturers that produce those rifles complete will require that you run full powered 5.56 NATO spec ammo to ensure reliability.  Those guns run like little race cars, under a lot of stress, at high RPM's and have reduced lifespans. 

 

So If you took a 16" barrel with a carbine length system (with .060 or .065 gas port) and cut it down to 10.5" and installed an adjustable gas port, you have to understand that you can not open it enough to make the gun run right, or at all. 

I understand that this is on the opposite end of the spectrum, but I am hoping this helps some to understand some of the basics of the gas systems and how they interact with different components.  When i'm tuning a gun, I can typically feel if its over gassed, not by the rise of the muzzle, but how the bolt feels during its cycle.  To me, there is a shock that vibrates the gun slightly, there are other variables as well, but the recoil impulse is lightening fast with an over gassed gun.  Yes, you can tone it down on any length system with an adjustable gas block, but there is a minimum amount of gas required to make the gun cycle and the closer the gas port is to the chamber, the higher the pressure requirement will be to facilitate that reliable cycle because its directly effected by the extraction process. 

 

 

 

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The devil is in the details.  You've pretty well covered the ground, but you left out a thing that is important for function of the gas system.  The different gas tube lengths have different volumes, and they admit gas to those volumes at considerable pressure differential to achieve the same level of bolt manipulation.  I'll put this in analogy form, though such is always suspect.  If these were springs, the pistol gas tube would be a very short, very stiff spring.  The carbine length is a bit longer, and not quite so stiff.  The progression continues with the mid-length and rifle versions of the tube.  The math gets very interesting when you start messing with gas flow and friction in these small vessels, but it is available in most of the physics books these days if you need to go deeper than this.

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Pretty sure I alluded to the volume thing right at the very first part of my first post. The water glass analogy. So maybe it was forgotten, but it sure was mentioned.

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