I have read some on these, and am trying to decide if I think there is a practical improvement with a mid length gas system. My carbine length gas system ARs seem to work fine. To those who own or have shot both, what do you think are the practical advantages and disadvantages of the mid length gas system?
Is a mid length gas system really better in a 16" AR?
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The theory is the mid has less pressure on the bolt and related parts as its unlocking and cycling. The on the ground facts are that the carbine gas system simply has far more actual, real life track record and development. The mid may never get the same amount of real life on the ground experience and research and development time to compare directly, so fans can just choose if they feel the theoretical difference is enough to be concerned over. I don't see any real disadvantage to mid, and like the theory, but to be brutally honest, its theory.
badkarmamib
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The carbine gas system was designed for 14.5" barrels. With 16" barrels, you have comparable dwell between the mid length gas port and the muzzle, functioning is about the same. However, moving the gas port to mid length allows the pressure in the chamber to drop more before extraction begins, increasing extractor and brass life.
I believe the carbine gas system was designed for a 10" then later adapted to an 11.5" barrel. The 14.5 " came about later.
And again, the advantages of the mid is still basically in the theory stage, until there are enough examples of the guns used side by side with carbine gas guns to get a true reliable comparison, which, based on the number of carbine gas guns in use over time, may never actually happen that we get a similar truly comparable track record for mids.
badkarmamib
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The biggest practical advantage is. . . longer stock handguards and sight radius. Honestly, there are studies showing a drop in chamber pressures when using a midlength, which might lead to longevity, but I have never experienced a noticeable difference in recoil when shooting my midlength vs. my wife's carbine.
Agree, I also like the longer handguards in standard format guns (I don't have any shaved FSBs), both the longer hand surface and the looks. I'm also a believer in the theory of mids, just wanted to comment on the actual reality of the comparison that's often not mentioned. They basically all work well, and do what Ive ever needed.
badkarmamib
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Oh, yeah, OP asked about disadvantages. Have you tried looking for aftermarket handguards for midlength compared to carbine or rifle? 
They are becoming more popular (because, as Malamute stated, they just look better with less barrel protruding past the handguard [subjective, I know]), but, it took me a while to find mine a couple of years ago.
They are becoming more popular (because, as Malamute stated, they just look better with less barrel protruding past the handguard [subjective, I know]), but, it took me a while to find mine a couple of years ago.It isn't theoretical physics we're talking about. The advantages are obvious and measurable. The problems with the shortest gas system are well known. And it is hard to see a disadvantage when the even longer rifle length gas system is known to be the most reliable.
badkarmamib
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Yes, rifle length gas is supposed to be the most reliable, but that is out of a 20" barrel. Too long of a gas system for barrel length will cause the pressure to drop, due to the bullet leaving the barrel, before cycling is complete. When the gas port diameter, buffer weight, buffer spring, BCG mass, ammo selection, etc. are correct, 14.5" carbine, 16" midlength, 18" intermediate length, and 20" rifle length are optimal, on paper. Most of us will spend more time figuring out which is the best system, than repairing the wrong choice.
primalmu
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I'm a fan of the mid-length gas systems. It may or may not provide any real world difference in terms of function, but I do prefer the longer handguard and sight radius. I feel that, at this point, it has been around long enough that we can safely say that from a reliability standpoint it is probably just as good as the carbine-length system.
lysanderxiii
Member
Actually, the 14.5 inch barrel was chosen for optimum performance with the arbitrarily picked carbine length gas system. The 10 and 11.5 inch barrels didn't work reliably without a moderator (which does nothing more than simulate a longer barrel.
There are a number of mathematical models of the M16 gas system, all of which show that for a 16 inch barrel the mid-length provides the lowest stress on the system while maintaining adequate bolt velocity with standard parts (ie, three steel weight buffer), over a wider range of port pressures.
That's not to say you cannot make a reliable CLGS work with a 16 inch barrel, it just means that with a mid-length, you are far less likely to find yourself over- or under- gassed out of the box.
For the gas system to be "too long", the barrel would have to be short for that length. In other words, the length of barrel in front of the gas block is what matters when it comes to pressure drop concerns, and a 16" mid has exactly the same amount of barrel in front of block as a 20" or a 14.5" rifle, so maintaining adequate pressure is obviously not a problem.
If you wanted to make a 40" barrel AR15, you'd obviously need a gas system of a custom length - but in picking a gas system for a barrel length between 14.5 and 20", it isn't hard to see what is likely to work well.
The real problem is that the AR15 bolt and bolt carrier is designed to open after its internal piston is moved a certain distance by a certain amount of pressure applied to it, and that pressure is based on a 20" barrel. So all shortened gas systems are going to open relatively early. The only truly "correct" way to design and short barrel AR15 is to change how far the carrier moves before it unlocks the bolt, which would restore the proper cyclic rate. Anything else you can do is just a bandaid to make up for the fact that the short barrel means you have a system pressure drop earlier than the rifle was designed for, so you have to move everything to an earlier point in the cycle to get things done. I know of at least one gun designer that did just that, but it requires a new bolt, carrier and upper to function.
Bottom line - if you have a 16" barrel, a mid gas tube is going to provide something closer to correct 20" gas pressure and timing than a Colt's XM177 gas tube length could.
I agree with all the theories of mid function, though its still theory until someone can make a large sample long term comparative study that shows improved bolt longevity etc. Just saying, lets be honest about what it is.
In theory, theory and practice are the same. In practice, they aren't.
I prefer 20" rifles most, mids next in preference, carbines last. Rifles are like 22s compared to the muzzle blast and choppy feel of carbines and wobbly adjustable stocks.
In theory, theory and practice are the same. In practice, they aren't.
I prefer 20" rifles most, mids next in preference, carbines last. Rifles are like 22s compared to the muzzle blast and choppy feel of carbines and wobbly adjustable stocks.
lobo9er
Member
Describe the theory? I am not an expert so as this may come off as sounding argumentative, I am really asking. I am asking because I would be willing to guess there is plenty of science and math to gauge gas pressure in impingement systems of any length, and if so there would be a very real measurement on wear and tear on parts. Not only would there be real measurements on gas pressure people/companies with knowledge on metallurgy would know how much wear and tear steel and alloys can take before they would be estimated to need replacement.
The theory basically goes as your posts supposes, that theres less pressure at the port, and less pressure on the bolt and carrier at unlocking and cycling, but there isn't any long term large numbers of examples side by side tests or hard data on exactly what the actual longevity difference is. That may be difficult to come up with because the mid is a civilian system with much more limited numbers of guns in use, of varying quality of parts (not made to TDP levels), and my point was it may never ever equal the amount of testing and real world experience that exists on the carbine length gas systems simply because the .gov/mil has done tons of work over time and we have hundreds of thousands of guns worth of real life experience to go on.
So yes, theres a lot of theoretical information, based on science and mathematical formulas etc, but its still theory compared to the sheer amount of hard data that exists on carbine systems. The only real way mid could catch up in hard data is if the .mil accepted it as a general use system, did years of research and field testing, and continually refined them for the next 20 or 30 years.
So yes, theres a lot of theoretical information, based on science and mathematical formulas etc, but its still theory compared to the sheer amount of hard data that exists on carbine systems. The only real way mid could catch up in hard data is if the .mil accepted it as a general use system, did years of research and field testing, and continually refined them for the next 20 or 30 years.
The "theory" is that when your gas system is short, the gas arrives at the bolt and opens it sooner. We know this because short gas system full auto ARs have higher cyclic rates. The short gas tube is also taping gas off earlier, so it is higher pressure, and it is dumping that pressure into a shorter, lower volume gas tube than the full size.
The rest of the theory is that the gas system is pressurized by the gas trapped between the breech and bullet as it moves down the barrel. If everything is designed right, the gas is delivered to the bolt carrier in time to open the bolt after the barrel is out of the bore, so chamber pressures are as low as possible. If the amount of barrel you have in front of the gas block is too much, then your bolt will be opening much closer to when the bullet is still in the bore and you chamber pressure really high. When that happens the bolt opens but gets a huge extra push from the chamber pressure making it "blow back". Extraction is supposed to be almost entirely from the bolt's movement, not from blowback forces.
A 16" barrel with a carbine gas system (a Frankenstein combination created to meet civilian barrel length minimums), has too much barrel in front of the gas block, so the bolt opens with too much residual chamber pressure, driving the bolt back even harder than what the gas tube pressure did (which was already a bit extra).
The Mid tube just gets the ratio of barrel in front of the gas block back to something closer to an M4 with a 14.5" barrel by putting the extra barrel length where it should be - between the gas block and receiver. This isn't tricky - once you understand what a terrible idea a 16" barrel with a carbine length gas system was.
Ideally, you'd design the bolt around the barrel and gas system length. Failing that, you would have a specific gas tube length for every barrel length. But we have three gas tube lengths and one bolt design. Some are obviously better combinations than others.
The rest of the theory is that the gas system is pressurized by the gas trapped between the breech and bullet as it moves down the barrel. If everything is designed right, the gas is delivered to the bolt carrier in time to open the bolt after the barrel is out of the bore, so chamber pressures are as low as possible. If the amount of barrel you have in front of the gas block is too much, then your bolt will be opening much closer to when the bullet is still in the bore and you chamber pressure really high. When that happens the bolt opens but gets a huge extra push from the chamber pressure making it "blow back". Extraction is supposed to be almost entirely from the bolt's movement, not from blowback forces.
A 16" barrel with a carbine gas system (a Frankenstein combination created to meet civilian barrel length minimums), has too much barrel in front of the gas block, so the bolt opens with too much residual chamber pressure, driving the bolt back even harder than what the gas tube pressure did (which was already a bit extra).
The Mid tube just gets the ratio of barrel in front of the gas block back to something closer to an M4 with a 14.5" barrel by putting the extra barrel length where it should be - between the gas block and receiver. This isn't tricky - once you understand what a terrible idea a 16" barrel with a carbine length gas system was.
Ideally, you'd design the bolt around the barrel and gas system length. Failing that, you would have a specific gas tube length for every barrel length. But we have three gas tube lengths and one bolt design. Some are obviously better combinations than others.
Can you describe why the 16" barrel with carbine gas is a terrible idea. Please do so in actual known results terms, not in theoretical advantage/disadvantage terms. I was of the impression they basically work pretty well. Perhaps not theoretically perfect, but in fact reliable with correct gas port size. Colt has been making them for about 35 or 40 years. Are they really noticeably, documentably less reliable than a 14.5" gun with the same gas system?
The "ideal" of designing different gas systems for every conceivable barrel length isn't likely practical from a major manufacturers standpoint, especially in military service. As you said, some are obviously better (in theory) than others, but they've come up with workable examples of all the common barrel lengths in service, without a different gas length for each. What I'm getting at, is the theory of different gas system lengths sounds good on paper, but in real life use, it hasn't proven to be needed. I think its more commercial market driven than actual need driven, but we get caught up in the hype and lose sight of whats been known for decades and in actual use. I like mid gas, but am not convinced any more that its a monumental leap forward. Just interesting.
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I believe that class III users probably could give you plenty of data on bolt head failures and feed jams with different configurations, but that isn't going to change the fact that a semi-auto AR15 will cycle and feed fine when relatively clean even when it is working well outside ideal parameters. It's only when you are trying to overcome dirt, or measuring parts life, or using less than ideal magazines that you begin to appreciate the difference between optimal and 'still working'.
These issues are real, and they are the reason the Marines adopted an HK to serve as heavy barrel carbine. In the real world, heavy volumes of fire demonstrate the problems with short gas systems when you change nothing else. If there was nothing to it, they just would have stuck a fat barrel into an M4 and called it a squad auto.
The entire history of the AR15 has been one where the designers came up with a well thought out system that was then grossly modified by Colt and others without going back to the initial design parameters and checking in the inventors. The original was designed with specific pressures, velocities and other expectations. Colt did everything through trial and error - with a lot of error.
As far as your question goes - sure, a 16" semiauto carbine works fine. Does a 16" mid work "better"? It will kick a little less, have a little less gas erosion and have a lower bolt speed with is less likely to feed jam. And a 20" rifle will be even better. All of those performance improvements aren't going to make your 3 gun match results different, so you can certainly say "who cares" if you'd prefer to. Some people like things to work as optimally as possible, and other folks like steel ammo and 3in1 oil. There is no wrong answer. At this point in time, I don't see why someone wouldn't use a now-common mid gas system for a 16" barrel AR - there certainly is no argument that the carbine gas system is better and a lot of hints that it is worse.
Plus, your bayonet lug stops being as stupid as it is on a carbine length 16".
lobo9er
Member
I agree with above statement. Also companies would also have hard facts/data on gas pressures on mid, carbine and rifle length impingement systems. I'm guessing big time company engineers are working with real numbers and not just guessing and theorizing on pressure. And if it is true that a system can be to long the the longer the system the less pressure there is. Only way to squash this one would be if there are any members here in the industry that are involved with the production of R's.
That's all true, but I think very little gun designing in the modern world is being carried out through the kind of engineering processes 1950s Armalite, FN or HK does. Most of what comes to the market today is trial and error, rules of thumb and copying previous working designs. AR's are easy to screw around with, and that's what companies do when they come up with AR derivatives.
Case in point: One of the machinegun ranges in Vegas came out with their maintenance observations, and the only piston AR design that actually worked and lasted was HKs, or the POF copy of the HK.
lobo9er
Member
True but we are talking about the impingement system. Manufactures are not reinventing the AR platform the way they had to engineer a piston system. lengthen it and it looses pressure, theres is a theory there I suppose, but also thats what happens. And its very easily observed for the most part. Less pressure equals less force on your bolt and so on, which would in fact lengthen the life of moving parts. All which is known. Unless I am way off. I will admit I know longer own an AR and have not shot one in sometime.
I don't believe anyone said there wasnt hard data on port pressure and such, just how much actual difference it makes in side by side comparison to carbine gas regarding longevity of parts and such. It may make sense that mid should last longer, but nobody has proven it with hard data of long hard use.
Regarding Battlefield Las Vegas, I dont recall that the piston HK was actually all that spectacular, perhaps just that that particular piston AR didnt crater like the other piston systems they used. I do also recall that the owner was very miffed when the barrel on the HK lasted about 10k rounds, which is WAY low on AR type guns they use. Battlefield Las Vegas would be a good test bed for various gas systems, but as I recall from reading the posts he made in arf and Mfork, they were getting WAY longer service life from all their guns than was generally thought practical, and as known replacement guidelines for .mil use. They had AR uppers and lowers going 200k rounds, and barrels that went I believe 100K rounds, with carbine gas bolts doing 30-50K rounds. In comparison, AK receivers started cracking at around 100K rounds in ways that they didn't want to repair them.
All of this is very interesting to discuss, but Ive come to the conclusion that some of this stuff just doesn't make all that much actual difference, and that there simply isn't the amount of research and testing, and real life use of any gas system that approaches the carbine gas system (perhaps rifle gas, but that's never really been considered an issue). Like it or not, it simply has the most work and actual field results to draw from. Its great to say "this or that is better", but until somebody proves it with anywhere near the same level of military level use and abuse, its all theoretical. That's all I'm saying.
To answer the question that's the topic of this thread, "Is a mid length gas system really better in a 16" AR?" Id say "probably, but so far nobody has actually proven it with hard data?"
The data points of the theory aren't the question, The actual real life effects/differences are the real question.
Some very qualified users had had higher end mids choke in use in hard use classes and competition, and they've used carbine gas M4s in .mil service for years and not had problems. I certainly cant argue with them about the theory of if mid is better or not, their real life experience hasnt proven it to them. I like mids, but I'm also not relying on one for my life either. most of use are not using out guns in life or death situations, our ideas of having one for home defense or whatever doesn't really put us in the same category of someone that uses one professionally to go shoot bad people in far away places.
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They aren't reinventing, but they are building on the non-engineering Colt did in the '60s when they made the XM177. We are still using that gas system, even though it was first made to work with a 10" barrel, then 11.5" when the 10" didn't work so well. Eventually every one of these rifles has a huge flashhider/suppressor thing to make them tolerable to use. Why are we using this gas system for a 16" barrel?
However, this is what the first Colt carbine length AR looked like with its 15" barrel. This rifle has essentially a mid gas system.
lobo9er
Member
I would challenge that, in a friendly manner of course. I would make the leap and say a manufacturer (if not many) some where at some time have torture tested both lengths and that data is logged somewhere. Less pressure and less force in any mechanism should equal to less wear and tear.
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