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AR-15 barrel length, gas operation/tubing, recoil discussion


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I’m going to nerd out here a bit in order to describe my understanding of the gas operation and but I was hoping to have a discussion about the effects of barrel length on recoil and the methods of reducing recoil. The general consensus as I understand is that a 18” rifle gas length AR is the lowest recoil barrel-gas system combination. It has a 12” port distance, with 6” of dwell length and 0.13ms of dwell time. Pressures  are 19.6ksi at the port and 14ksi at the muzzle. I would consider this the ‘working pressure’ range that is used to operate the carrier during the pressure stages when it is accelerating. Generally the carriers have an operating internal gas pressure of 2.2ksi (and an area of ~0.2in^2) and reach a velocity of 15ft/s. So, there’s a significant reduction in gas pressure between the port and carrier chamber even without an adjustable gas block. The rifle I’m trying to reduce recoil on has an 11.5” barrel with 7” carbine gas length, so 4.5” dwell length and 0.13ms dwell time but pressure of 33ksi and 22.6ksi for port and muzzle. As I understand it, the bullet leaves the barrel and gas pressures decrease before any carrier movement really occurs, and you certainly don’t have unlocking; I’m basing this off slow motion video and the relative velocity of the carrier to bullet (~0-15ft/s vs 2500+ft/s). The timing effects of the gas tube length don’t seem important because of how fast the pressure moves, even relative to the bullet speed and certainly the carrier. That acceleration of the carrier has to occur during the dwell length/time of the bullet between the port and muzzle, which for these two barrels are the same. With an adjustable gas block to act as a restrictor and decrease the gas pressure to the carrier, and similar dwell times, I don’t see how the gun would behave any different between a carbine 11.5” and a rifle 18”. The extra gas pressure at the muzzle certainly could contribute to recoil, however the bullet is also going slower; furthermore, more gas pressure helps operate a muzzle device (like 9mm minor vs major with a compensator). 

 

Then you can further do all the standard tuning to reduce the parasitic loss of energy from friction (carrier, bolt, feed lips and feed ramp polishing), followed by moving mass/spring ratio/gas block adjusting) to have the bolt hold open on last round but not have the buffer bottom out on the extension tube. With all of this done, why would the recoil be any more (if anything, less) than an 18”? I had a suppressed 10.3” that was harsh on recoil but was obviously overgassed. 

 

This link (https://www.ar15.com/forums/ar-15/How-it-Works-The-AR-gas-operation-and-how-everything-works-in-harmony-/66-266108/) has a great explanation of the gas operation but one thing I don’t understand is how the carrier and bolt can already begin to be extracting a round before the bullet leaves the muzzle (in an overgassed situation). The symptoms seem to agree with it but it is surprising; that’d have to move ~1/2” at an average velocity of ~100in/s whereas the bullet has to move 4” while it’s going ~30,000in/s, which is an order of magnitude more time...

 

Thoughts, experiences? 

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I can tell you that an 18" rifle vs. 16" interm. gas "feels" softer to shoot. The recoil to me on an 18" seems to be a"longer" push and not as short/snappy as 16". Now saying all this, a 223 has very little recoil anyway so its kinda subjective. I have a few sbr's and they is noticeable more snap to them but they are 10-12" also. 

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1 hour ago, kurtm said:

Restriction does not decrease pressure. The closer you are to the chamber the higher the pressure.

Huh, that’s interesting. This mil report (BRLR 1475) seems to show the bolt carrier pressures only hitting about 2.5ksi, which seems like there’s expansion and restriction of that metered gas to a lower pressure than the chamber pressure. I’d think further restriction by a restrictive adjustable gas block would lower that carrier pressure more?

4BA9866F-E46E-4185-B691-B4D12EE0F57C.jpeg

 

From another article showing gas port size (restriction) effects on carrier chamber pressure: 

 

2CB3D7C9-7735-4D89-8765-D80ADD5B3537.jpeg

 

Maybe @TonytheTiger has some input on this?

Edited by Benevolence
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On 2/11/2021 at 2:25 PM, Benevolence said:

Maybe @TonytheTiger has some input on this?

You've already gone way deeper on the numbers than I ever have. I've done a lot of testing with various barrel lengths, gas system lengths, reciprocating weights and spring rates, but I've never put a lot of time into studying graphs or doing any math to understand exactly what's going on. 

 

The biggest takeaway from all my tinkering is that the shorter the gas system length the harsher the impulse will be. AGB's help tame a system in any given length, but longer will always end up more docile. Dwell time seems to have some effect on felt recoil. For instance, a carbine gas 16" barrel feels a little more violent (not taking muzzle blast into account) than a carbine gas 12.5" with all other components identical. Same gas system length, very different dwell times. But it doesn't work the other way. A 16" rifle gas may have similar dwell time to a 10.5" carbine gas but that in no way equals a similar recoil impulse.

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7 hours ago, TonytheTiger said:

You've already gone way deeper on the numbers than I ever have. I've done a lot of testing with various barrel lengths, gas system lengths, reciprocating weights and spring rates, but I've never put a lot of time into studying graphs or doing any math to understand exactly what's going on. 

 

The biggest takeaway from all my tinkering is that the shorter the gas system length the harsher the impulse will be. AGB's help tame a system in any given length, but longer will always end up more docile. Dwell time seems to have some effect on felt recoil. For instance, a carbine gas 16" barrel feels a little more violent (not taking muzzle blast into account) than a carbine gas 12.5" with all other components identical. Same gas system length, very different dwell times. But it doesn't work the other way. A 16" rifle gas may have similar dwell time to a 10.5" carbine gas but that in no way equals a similar recoil impulse.

Thanks; I’ve read a fair amount and your posts seem to have some good content so I was hoping to get your thoughts. Also how easy it is to be flat out wrong with over analyzing things without actually trying it and overlooking things that are important. Just trying to understand the recoil from a fundamental perspective. 

 

I’ve got a 11.5” carbine (Spikes) JP SCS with white spring and thinking about replacing the steel weights with a Teflon spacer. That, and milling lightening cuts in a F/A carrier and putting in a POF cam pin. Hoping to use the AGB in conjunction with it to get the recoil tuned down. I guess I can math all day but it comes down to actually just trying things out. 

Edited by Benevolence
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We have 3 different 16" barrels.

 

Mine is a Spec Ops 16" rifle length with their angled gas port non adjustable gas block, JP SCS stock set up, low mass JP BCG. RCA adjustable gas key, Surefire comp.

Granddaughters, Criterion 16" rifle length gas, non adjustable gas block, low mass BCG, old Taccom UL buffer, RCA AGK, Lantac dragon comp.

Sons, Stretch 16, non adjustable gas block, Taccom buffer, RCA AGK, Seekins low mass BCG & comp.

 

All 3 have CMC triggers. Gun weights are about the same. Gas adjustment are all set high for reliability. 

 

After shooting all in various matches and comparing pulse, recoil, accuracy past 300 yards, believe it or not they are all very similar. Granddaughter's is probably the softest shooting most accurate with mine next. Stretch runs 3rd. All 3 are super reliable.

 

Not sure on the math, but real world. Have had and shot over the years every type barrel from carbine length, mid, to rifle and longer gas system is the way to go.

 

gerritm

 

 

 

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Boyle was a much smarted dude than I, but

 

I think you are confusing pressure with volume when you are referring to an adjustable gas block. Ever put your thumb over a garden hose to reduce the flow of water but make it shoot farther? Thats how an adjustable gas block works......sorta. With gas (as opposed to liquid witch is not compressible) it is expanding to a certain point and in an un regulated system we get much more expanding gas (Volume) than needed. An adjustable gas block just allows us to reduce the volume of expanding gas to a more tolerant level as it pertains to recoil.

 

The other part of the equation is weight (mass) of the barrel. All things being equal a longer (more mass) barrel will be more resistant to the effects of the carrier bouncing around and result in less felt recoil (I know there's another law of physics in there as well, just can't remember witch one). That alone means there is no way, all things being equal, to make that 11.5" bbl recoil equivalently to an 18". 

Edited by mpeltier
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Gas, in this case, is quite a misnomer. The "gas" from a modern propellant acts more like a plasma. Which at higher pressures is not very compressible. Constricting a plasma increases it's velocity.....much like the garden hose analogy, but does not drop the volume like it would if it were a "gas". It still effects volume to some degree, but what you are doing in reality is increasing the velocity of the plasma in the same space as before i.e. gas tube volume, bolt carrier volume, etc. More plasma velocity equals quicker carrier reaction. Yes you can constrict the plasma to a point that it doesn't have the volume to operate in the gas tube and carrier, but if it is constricted to the point it just nearly operates it will operate faster the closer you are to the peak pressure found at the chamber. The longer the gas tube the more you slow the plasma collum due to friction of the tubing and the volume of the tubing....this slowing the velocity of the plasma hitting the carrier.....less velocity = less violent carrier motion, but hey graphs and theoretics based on a true gas make for great table talk even when it really doesn't apply to the topic at hand.

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5 hours ago, kurtm said:

Gas, in this case, is quite a misnomer. The "gas" from a modern propellant acts more like a plasma. Which at higher pressures is not very compressible. Constricting a plasma increases it's velocity.....much like the garden hose analogy, but does not drop the volume like it would if it were a "gas". It still effects volume to some degree, but what you are doing in reality is increasing the velocity of the plasma in the same space as before i.e. gas tube volume, bolt carrier volume, etc. More plasma velocity equals quicker carrier reaction. Yes you can constrict the plasma to a point that it doesn't have the volume to operate in the gas tube and carrier, but if it is constricted to the point it just nearly operates it will operate faster the closer you are to the peak pressure found at the chamber. The longer the gas tube the more you slow the plasma collum due to friction of the tubing and the volume of the tubing....this slowing the velocity of the plasma hitting the carrier.....less velocity = less violent carrier motion, but hey graphs and theoretics based on a true gas make for great table talk even when it really doesn't apply to the topic at hand.

Good explanation of one of the reasons a longer gas system is softer shooting than a short gas system. 

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If a longer gas system is the answer to softer shooting, then what is the result of gas tubes with a loop or two?

Sounds like a simple modification without the added weight of a longer barrel.

   Mark

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If I understand correctly, you’re saying the gas behaves incompressible until it begins expanding in the carrier, and restricting at the gas port only increases the velocity, decreases the delivered mass and doesn’t affect the pressure of the gas. And that the benefits of the gas length are two fold; decreased port (and delivered) gas pressure and velocity into carrier. 

 

I suppose the Fat Boy or (perhaps more gimmicky) pig tail gas tubes could be an option. 

 

Oh, and I don’t really see how measurements of pressure at the ports and carriers are irrelevant or table talk; I was just trying to present those as they seemed relevant and to demonstrate that there was a pressure decrease in the carrier chamber vs port. I thought it was due to pressure losses along the way and hoping to understand the gas pressure/velocity in relation to the bullet position relative to the gas port and muzzle. 

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The "gas" isn't a gas is what I am saying. It is gas with solids, particulates, and liquids all flowing together, made up of all sorts of chemical compounds. So looking at perceived "gas" charts based on a pure gas.....not a plasma, don't give an accurate rendition of what is happening......and yes pig tails help, but once again, with feeling, the closer you are to the chamber with your "gas" port, the higher your "gas" velocity, the more violent/faster the carrier reacts.

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