Benevolence Posted February 11, 2021 Share Posted February 11, 2021 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? Link to comment Share on other sites More sharing options...
donno Posted February 11, 2021 Share Posted February 11, 2021 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. Link to comment Share on other sites More sharing options...
kurtm Posted February 11, 2021 Share Posted February 11, 2021 Restriction does not decrease pressure. The closer you are to the chamber the higher the pressure. Link to comment Share on other sites More sharing options...
Benevolence Posted February 11, 2021 Author Share Posted February 11, 2021 (edited) 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? From another article showing gas port size (restriction) effects on carrier chamber pressure: Maybe @TonytheTiger has some input on this? Edited February 11, 2021 by Benevolence Link to comment Share on other sites More sharing options...
kurtm Posted February 12, 2021 Share Posted February 12, 2021 Maybe the laws of physics has some input on this. Calling Mr. Boyle? Link to comment Share on other sites More sharing options...
TonytheTiger Posted February 13, 2021 Share Posted February 13, 2021 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. Link to comment Share on other sites More sharing options...
Benevolence Posted February 14, 2021 Author Share Posted February 14, 2021 (edited) 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 February 14, 2021 by Benevolence Link to comment Share on other sites More sharing options...
gerritm Posted February 14, 2021 Share Posted February 14, 2021 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 Link to comment Share on other sites More sharing options...
TonytheTiger Posted February 14, 2021 Share Posted February 14, 2021 15 hours ago, Benevolence said: I guess I can math all day but it comes down to actually just trying things out. Yup, that's where the rubber really hits the road. Link to comment Share on other sites More sharing options...
mpeltier Posted February 18, 2021 Share Posted February 18, 2021 (edited) 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 February 19, 2021 by mpeltier Link to comment Share on other sites More sharing options...
kurtm Posted February 19, 2021 Share Posted February 19, 2021 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. Link to comment Share on other sites More sharing options...
mpeltier Posted February 19, 2021 Share Posted February 19, 2021 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. Link to comment Share on other sites More sharing options...
mpom Posted February 21, 2021 Share Posted February 21, 2021 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 Link to comment Share on other sites More sharing options...
Benevolence Posted February 22, 2021 Author Share Posted February 22, 2021 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. Link to comment Share on other sites More sharing options...
kurtm Posted February 23, 2021 Share Posted February 23, 2021 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. Link to comment Share on other sites More sharing options...
K-Tex Posted October 14, 2021 Share Posted October 14, 2021 One reason I work off of volume and not pressure, especially in less than 16" barrels. Link to comment Share on other sites More sharing options...
kurtm Posted October 14, 2021 Share Posted October 14, 2021 Just like a Thanksgiving dinner......you gotta work off volume! Pressure is released by the "gas tube", but the volume stays and stays Link to comment Share on other sites More sharing options...
K-Tex Posted October 15, 2021 Share Posted October 15, 2021 Easy way on figuring time in barrel. velocity 3,000fps barrel length 24" 1 / 3,000 x 2= .00066+ of a second. 18" barrel w/same velocity 1 / 3,000 x 1.5=.0004999 Doesn't account for acceleration from ignition but close. Link to comment Share on other sites More sharing options...
K-Tex Posted October 15, 2021 Share Posted October 15, 2021 On 2/11/2021 at 2:25 PM, Benevolence said: 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? From another article showing gas port size (restriction) effects on carrier chamber pressure: Maybe @TonytheTiger has some input on this? Gas poet is an orifice so ir will restrict gas volume and pressure (adjustable can further reduce it). On ordering custom barrel thay ask if an adjustable gb will be used? If yes or no (yes gp at .100 and no at .080"), so they use a larger port size for adjustable gb. Different companies might use different sizes based on gas system length? Link to comment Share on other sites More sharing options...
kurtm Posted October 16, 2021 Share Posted October 16, 2021 So how much gas volume do you have in a less than 16" barrel. Is it different for a 16" barrel or a 20" barrel? If so by how much? Is gas volume dependent on the powder and primer in any given cartridge? Link to comment Share on other sites More sharing options...
K-Tex Posted October 16, 2021 Share Posted October 16, 2021 Maybe this can help us both? http://ar15barrels.com/making.shtml Link to comment Share on other sites More sharing options...
K-Tex Posted October 16, 2021 Share Posted October 16, 2021 It's not 100% definitive! Link to comment Share on other sites More sharing options...
K-Tex Posted October 16, 2021 Share Posted October 16, 2021 It also doesn't address how headspace is done only which stage of process. Link to comment Share on other sites More sharing options...
kurtm Posted October 16, 2021 Share Posted October 16, 2021 Wrong place, this is the volume thread, the one where you state you always work off volume. If one works off volume all the time and most of the components that make up an upper are constants, I would suspect one might be able to come up with measurements on volume, so what is the volume of gas in a less than 16" AR-15? I'm guessing there should be a number in cubic inches or any other type of volume measurement you wish, but it should be a number. Also if one is using volume one should be able to tell us how much volume of gas is created when a cartridge is discharged, once again a number, and they should be able to tell if that volume would fluctuate cartridge to cartridge. Link to comment Share on other sites More sharing options...
kurtm Posted October 17, 2021 Share Posted October 17, 2021 Still waiting on your numbers. We will accept any cubic meter, cubic millimeter, or centimeter, cubic inch, yard, old French pounce........ or you have no idea of what your talking about, which we learned talking about barrel extensions. Expertise by DELL. Link to comment Share on other sites More sharing options...
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