Is a mid length gas system really better in a 16" AR?

[lobo9er]

"but with different gas systems—the longer gas tube will produce a reduced dwell time. The practical results are usually slower bolt-carrier movement and a slight reduction in felt recoil. The latter facet is quite apparent to someshooters but can go completely unnoticed by others. I regularly shoot all three gas-system lengths, and I can feel the difference in otherwise-like barrels."

https://www.shootingillustrated.com/articles/2014/9/24/ar-gas-system-lengths-explained/

heres an interesting article not sure its an end all to our discussion but interesting. the cool thing about threads like these is that I end up reading and learning more about a topic that I more than likely wouldn't have bothered to read up on otherwise.

 

[Malamute]

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.

If such exists, and I doubt it does, I think it would be put forward in the marketing of the mid length guns.

I agree that "should equal less wear and tear" is the presumption, we just don't have the hard proof so far, and I doubt we ever will, at least not in the foreseeable future. Mids sell well, are popular, and generally accepted. Most don't need hard data, I'm just saying we've accepted as proof whats really a theory, and so far we only really have scattered anecdotal information. If someone takes 50 or 100 of each gun, identical in every way other than gas length, and shoots them all the same amount with the same ammo in the same conditions, and measured all the wear parts before, during and after, we would start to have hard data. The cost of that would be tremendous, just in man hours, not to mention guns and ammo. Its just never been needed to sell mid guns. If the .mil ever actually becomes interested, we would also start to get serious real data. I doubt they will, they are already heavily invested in M4s and Mk18's with carbine length gas systems, but would welcome it if they chose to.

 

[RX-79G]

If such exists, and I doubt it does, I think it would be put forward in the marketing of the mid length guns.

When the present day Armalite came out with the mid, they did just that. But the kind of testing you're talking about isn't necessary to market the (fairly obvious) benefits, so I don't know if anyone has bothered to compile it. Colt may have their old data, too.

But really, no one is making just Mid anything, so they don't have to market it. They are more likely to market how they give you the option of three lengths than market against the more common carbine length.

 

[MachIVshooter]

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.

Your information is about 15 years outdated. Midlength is far and away the more popular system now, for all the reasons mentioned in this thread, and thus it has been quite well vetted in the real world as we roll up on 2017. Bottom line, lower port pressure and proper dwell time made the mid length a less violent system than 16" tubes with carbine gas. I have carbine gas uppers, but they have barrel lengths suited to that system; 10.5", 12.5", 14.5". I have owned 16" guns with carbine gas, and never will again.

Besides which, 16" M-4geries look retarded.

Mark Westrom did the AR world a huge favor 20 years ago. It just took people a little while to realize that.

 

[azrocks]

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.

The 'tons of experience' that you cite relating to the carbine-length gas system cannot be divorced from barrel length (or other factors like gas port size, etc). So yes, we know ALOT about how a carbine gas system works with 14.5" barrels common to the M4, but that doesn't translate to how it works with a 16" barrel, which is what the OP was asking about.
The gas system length is but one factor in the equation. You can't just compare one length system to another without accounting for everything else involved (gas port size, carrier mass, buffer weight, etc, etc).

If you suggest - as apparently you do - that a carbine-length gas system is a proven winner with 14.5" barrels, then logic dictates that when you increase barrel length over the same gas system, the tuning is likely going to become sub-optimal. If it was 'perfect' at 14.5", it's going to be somewhat over-gassed at 16". We may argue as to the extent of that over-gassing, and whether it ultimately makes a significant difference in terms of reliability, component wear, etc... but it exists. If you take a well-running 14.5" carbine-length gas system rifle and do nothing more than add 1.5" to the barrel, at the very least you'll increase felt recoil for no gain whatsoever. Most likely, however, you'll also contribute to accelerated parts wear, increased fouling, and potential malfunctions with hot ammo.

Middies for 16" barrels are where it's at. That's something the guys making rifles for the US military already know.

 

[lysanderxiii]

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.

Not quite, increasing the mass of the carrier/buffer combination changes when the bolt unlocks by slowing down the acceleration, even with the same amount of travel before unlocking. Because of the very short time intervals involved, a few thousandths of a in/sec/sec in acceleration makes a big difference in timing.

Also, that rather simplistic sketch fails to take into account of the reduced gas flow from a smaller orifice (gas port) which reduces the mas flow to carrier, and so reduces the speed at which the pressure in the carrier rises.

 

[RX-79G]

The problem with screwing around with port size is that you don't get something for nothing - a smaller port will still let the gas through, but now the gas has to move faster through the reduced port, which is going to be erosive and cause the gas to cool a bit just on the other side as it expands. I'm not saying it isn't a tool in the toolbox, but it has its own downsides. It's a bit like how pigtails don't do as much as they seem like they should - gas isn't so much traveling as pressurizing the system.

Increasing the mass is a good idea - unfortunately the move from rifle to carbine is usually a net reduction in mass.

The "perfect" 16" carbine is going to take advantage of all the off-the-shelf things you can do. Or, you can do none of them because they all increase the weight and you wanted a light rifle that is reliable enough.

 

[Malamute]

Posts No 29 and 30, I don't think you are really reading carefully what Ive written. I still see no actual hard evidence that mids last any longer, are more reliable or have the actual round counts to compare with the level or use and research that has gone into carbine gas guns. I at NO time said carbine was perfect in any way shape or form, that is a reading comprehension difficulty on someones part. I do see people reading into my comments what they wish to, to reinforce their confirmation bias or whatever. Apparently the part was missed where I said I liked mids and believe the general theory, but the need to go on the defensive of the mid concept is too great to resist trying to make out what Ive said into something I did not.

I had a feeling things would go the way they have. The funny part is I used to be one of the staunch defenders of the concept, until what others were saying started to sink in and I realized it was true. It takes nothing from the mid concept, other than the sense of superiority of system some seem to feel trumps actual use. Mid is a civilian product. Unless and until its used in military service and racked up some real on the ground hard use and abuse, it just isn't going to have the same level of vetting that the existing .mil guns have. Reading anything more than that into what I wrote is a breakdown of reading comprehension.

 

[Tirod]

Old conversation for those of us who frequent an AR oriented forum.

When the M4 was copied for civilian sale it required an NFA legal 16" barrel, and the easiest cheapest way to do that was simply machine everything with a longer length of barrel in front of the gas block to use the standard short handguards and tube. It was not following the engineering reality of the operating system, tho. The weapon works on a certain optimal range of pressure to dynamically cycle the bolt. Not really any different than the auto engine - too much pressure and it pushes the limits of mechanical endurance. Carbine gas on a 16 does that to an incremental degree more than mid length.

The proof of the "argument" is that once M4gery sales were going strong, makers saw their QC issues rise and it was directly related the higher pressure of carbine. Their answer was to promote midlength, which was the correct port location for a 16" barrel.

At first not many were interested but as time went by there were increasing numbers of guns with issues and then AR fans began looking into the difference. With more reports of damage leaking from makers, then the growing number of guns with catastrophic failures, some started to photograph the action while moving with high speed cameras. What they found was surprising, as the carbine gas system had a higher speed while cycling which actually caused the bolt to bounce off the barrel extension. If the primer was struck and the powder burning while the bolt unlocked, the cartridge would extract just enough so that the weaker portion of the brass was pulled from the chamber and the wall would fail. Gas pressure at about 55,000 pounds would escape from the cartridge into the action which causes violent damage to the upper, blows out the magazine, and breaks the bolt.

Check the dates on Kaboom reports - as midlength gained ground in the market the number of incidences fell. It wasn't high, and not every incident was because of carbine gas, but that was the reason the majority of companies switched.

I'll take their word for it - midlength works better. The proper position for a gas port is located about five inches from the muzzle - take a look at where it is on rifle and carbine on the GI guns. As the barrel gets shorter gas pressure increases - it's about double on a 10.5" compared to 20" - and the distance to the muzzle shortens in a calculated manner. Carbine gas on a 16" is the anomaly and that's because it was just doing it for style, not because it was right.

Old information, it's been going on since the XM177 was approved back in 1965. The M16/AR15 work off of gas pressure and timing, they have optimal values for each barrel length and aren't interchangeable.

 

[azrocks]

Posts No 29 and 30, I don't think you are really reading carefully what Ive written. I still see no actual hard evidence that mids last any longer, are more reliable or have the actual round counts to compare with the level or use and research that has gone into carbine gas guns. I at NO time said carbine was perfect in any way shape or form, that is a reading comprehension difficulty on someones part. I do see people reading into my comments what they wish to, to reinforce their confirmation bias or whatever. Apparently the part was missed where I said I liked mids and believe the general theory, but the need to go on the defensive of the mid concept is too great to resist trying to make out what Ive said into something I did not.

I had a feeling things would go the way they have. The funny part is I used to be one of the staunch defenders of the concept, until what others were saying started to sink in and I realized it was true. It takes nothing from the mid concept, other than the sense of superiority of system some seem to feel trumps actual use. Mid is a civilian product. Unless and until its used in military service and racked up some real on the ground hard use and abuse, it just isn't going to have the same level of vetting that the existing .mil guns have. Reading anything more than that into what I wrote is a breakdown of reading comprehension.

There's nothing wrong with my reading comprehension, but there may very well be something wrong with your powers of deduction.

You don't need real-world vetting to understand that if moving components in any mechanical system are exposed to increased heat, force, & pressure, their lifespan will be impacted and their potential for premature failure increased. Nor do you need real-world examples to prove that increasing the barrel length over an existing gas system will increase gas volume and pressure... it's fairly simple math. Nothing on the battlefield that I'm aware of is going to alter the laws of physics.

If you were willing to completely dismiss all the facts Tirod provided, you might argue - based solely on a lack of definitive proof - that adding another 1.5" to a 14.5" carbine might aid in cycling & extraction (it doesn't, but you could argue that). But there's no way you can argue it's not going to generate more wear & tear on the rifle. Once again: more heat, more pressure, more force. Those are physical realities you can't get away from.

 

[Malamute]

I actually agree with pretty much all you and tirod posted, my ONLY contention is we don't have actual, solid, verifiable data on how much difference it actually makes, as mid is a commercial creation, used by a bunch of various makers with different specs of parts (despite being called "mil-spec", not all parts are actually made to true TDP specs and materials/hardening/testing etc). Tirods post was information, but not proof or data. I'm not dismissing any of it, just stating the fact that mids don't have the amount of data on actual use that gives definitive values on how much difference it actually makes, such as average bolt life to failure, gas port life to failure, etc. If youre reading more than that into my posts, youre missing or adding something.

Saying "mid length configuration obviously has less pressure etc, isn't "data". It may be a fact, but doesn't make the result of the conclusion that its based on "data". X number of average rounds between bolt or port failures between different systems with statistically valid numbers of guns, bolts and rounds is data, anything less is not data.

Saying anything about mid in my direction that isn't hard data about statistical, validated improvements in parts longevity isn't telling me anything. I researched it for a year or so, reading all I could find on it. Nothing posted here is the slightest bit new to me, other than somes contention that actual data exists about improved parts life with mids, because I haven't seen any hard data, and nobody has presented any actual data. I believe its a great idea, but what the majority of posts here seem to be doing is trying to convince me that I obviously don't know about it or understand its advantages, which is all incorrect. Totally incorrect.

Id be glad to see the data, as I believe the theory to be valid, but so far, here or anywhere else, Ive not seen any true, real life data that conclusively proves that the overall parts longevity is a real, provable thing, not a theory.

 

[Varminterror]

There is a lot of conjecture being thrown around in this thread with very little understanding of internal ballistics, pressure curves, or basic physics.

It's not a complicated system. Anyone with $150 for Quickload can draw up the appropriate pressure curve, and anyone who finished high school physics can determine the resulting forces kicking around the action. Given an existing system, this is all a person needs to run the numbers on the action.

The engineering work to determine appropriate port size and pressure drop across the gas tube, the transient state operation of the opening bolt and declining pressure over dwell time is well within the reach of anyone who finished their freshman year of engineering school (college physics and calc 1 & 2). This kind of person could design the system from scratch - which is to say the folks who came up with it 60yrs+ ago aren't so unique in their ability to run numbers.

What I can say, after working with some firearms design firms in my own professional engineering career, is the engineers today have tools on hand which can do engineering design computation within minutes and seconds which would have taken dozens if not hundreds of hours of hand calculations. There IS a downside, of course, meaning lower quality engineers can accomplish far more than higher quality engineers of yester-year, but at the same time, it also means even the slack-jawed intern fresh out of college can run computations and calculations which were effectively prohibitive even for the top minds a few decades ago.

The good news - however, is that fixing the issue is incredibly simple. Want the performance of a mid-length but have a carbine length? Throw on an adjustable gas block and you can throttle the pressure down in a carbine system to the same level as a mid. Want the same bolt speed and cyclic rate as a rifle length system out of your carbine? Throw in an H3 buffer.

But the real answer is this: There are multiple combinations which operate with high reliability and extreme longevity. Not all AR's have to have the same bolt speed and cyclic rate as a 20" M16A1. Ford F-150's make it down the road even though they might offer 3 different motors in the same body class, and all have their own specific attributes. AR's will reliably operate with multiple different gas system combinations.

 

[RX-79G]

There is a lot of conjecture being thrown around in this thread with very little understanding of internal ballistics, pressure curves, or basic physics.

It's not a complicated system. Anyone with $150 for Quickload can draw up the appropriate pressure curve, and anyone who finished high school physics can determine the resulting forces kicking around the action. Given an existing system, this is all a person needs to run the numbers on the action.

The engineering work to determine appropriate port size and pressure drop across the gas tube, the transient state operation of the opening bolt and declining pressure over dwell time is well within the reach of anyone who finished their freshman year of engineering school (college physics and calc 1 & 2). This kind of person could design the system from scratch - which is to say the folks who came up with it 60yrs+ ago aren't so unique in their ability to run numbers.

What I can say, after working with some firearms design firms in my own professional engineering career, is the engineers today have tools on hand which can do engineering design computation within minutes and seconds which would have taken dozens if not hundreds of hours of hand calculations. There IS a downside, of course, meaning lower quality engineers can accomplish far more than higher quality engineers of yester-year, but at the same time, it also means even the slack-jawed intern fresh out of college can run computations and calculations which were effectively prohibitive even for the top minds a few decades ago.

All of this is true about engineering software, but none of it actually has much to do with anything discussed in the thread. You've said that the participants in the thread have "very little understanding". Could you please relate that to something, rather than just imply that people don't understand.

 

[Varminterror]

Could you please relate that to something, rather than just imply that people don't understand.

Here is ONE example, since you asked - Tirod - please don't take this as an attack, but it's a plain and simple example of a lack of understanding. Which, admittedly, is one of the most commonly overlooked issues in any "AR-15 design" discussion - the mysterious gas port...

When the M4 was copied for civilian sale it required an NFA legal 16" barrel, and the easiest cheapest way to do that was simply machine everything with a longer length of barrel in front of the gas block to use the standard short handguards and tube. It was not following the engineering reality of the operating system, tho. The weapon works on a certain optimal range of pressure to dynamically cycle the bolt. Not really any different than the auto engine - too much pressure and it pushes the limits of mechanical endurance. Carbine gas on a 16 does that to an incremental degree more than mid length.

Change the diameter of the gas port and all of the above conjecture about NFA compliance and the 16" barrel vs. 14.5" using the same gas system length becomes moot. A smaller port on a shorter Carbine system can actually expose the action to LESS pressure than a larger port on a Mid-length system.

The arguments "the rifle was designed for the pressure 20" barrels with 12.5" gas systems so carbine stocks need to have H3 buffers" or the "M4 was designed with a 14.5" barrel and a 7.8" gas system so a civilian M4 clone is over gassed" are all completely arbitrary - rifles can operate safely, reliably, and with high longevity with a wide range of operating conditions, so a wide range of spring, port size, buffer/carrier weight, and gas system length will work just as well as another.

Are there advantages to a mid-length system? Sure. Are there disadvantages? Eh, not many.

Personally, I'd rather have a carbine length system with an AGB than a mid-length system. I can always throttle down, but you can never throttle up without reaming the port, and even then, it's a limited game.

 

[RX-79G]

Change the diameter of the gas port and all of the above conjecture about NFA compliance and the 16" barrel vs. 14.5" using the same gas system length becomes moot. A smaller port on a shorter Carbine system can actually expose the action to LESS pressure than a larger port on a Mid-length system.

Sure. But an engineering program isn't going to show the increased port erosion or increased gas tube fouling from restricting the port. You have to be able to factor in the nozzle effect.

If every AR issue could be solved by simply restricting the port size, then there would be no AR issues.

 

[Varminterror]

You obviously haven't worked with many engineering design programs. Fouling factors and port erosion factors have been "built ins" for a very long time in flow system design software.

 

[RX-79G]

So when you used them, did it help you understand why military rifles with adjustable gas systems don't work by restricting gas? They work by dumping the extra gas.

 

[Malamute]

I don't believe the pressure is reduced with a smaller port, the pressure is the pressure. Volume will change, but not port pressure. Shorter length gas system means higher gas temps and higher port pressure,and the port size has been developed to function reliably with a variety if environmental conditions, and the end result of buffers, springs and such ends up with a workable system for the given length. Port erosion will generally (or theoretically ) be greater with a shorter system at higher pressure. I believe this is fairly well understood, but have no hard information about how much it affects it. How much difference it makes in the life of the barrel in the greater scheme of things is what I don't know.

 

[Varminterror]

So when you used them, did it help you understand why military rifles with adjustable gas systems don't work by restricting gas? They work by dumping the extra gas.

Not surprised to find more closed minded folks here.

I've been making "military rifles" work by restricting gas for over 15yrs, pay uncle sam every year for the privilege of operating that business on American soil. If properly matched gas flow and carrier/buffer weights didn't work, I wouldn't have people coming back for more.

 

[Varminterror]

I don't believe the pressure is reduced with a smaller port, the pressure is the pressure. Volume will change, but not port pressure.

Then you believe wrong.

Restrictions in pressure systems create "pressure drop."

 

[RX-79G]

Not surprised to find more closed minded folks here.

I've been making "military rifles" work by restricting gas for over 15yrs, pay uncle sam every year for the privilege of operating that business on American soil. If properly matched gas flow and carrier/buffer weights didn't work, I wouldn't have people coming back for more.

And I could accuse you of defending your vested interests. But why don't we just discuss the topic?

Why do you think that military rifles with adjustable gas systems don't work through restriction?

 

[carbine85]

I don't care about the theory. I know for fact that my mid lengths run smoother than my carbines. Flying by the seat of pants I choose the mid length over the carbine.

 

[Malamute]

I don't care about the theory. I know for fact that my mid lengths run smoother than my carbines. Flying by the seat of pants I choose the mid length over the carbine.

Nothing wrong with that.

 

[Malamute]

Then you believe wrong.

Restrictions in pressure systems create "pressure drop."

OK, if thats not a correct assumption on my part about the pressure not being reduced, if thats an unqualified improvement, why wasn't it used to reduce gas pressure in the M4s? I'm asking if its a definite improvement in overall function and reliability. If making the port size of an M4, or using an adjuster to deliver the same pressure to the carrier as a 20" rifle, it should offer the same function, but theres something more to it than that. They didn't do that for some reason. For a civilian use gun, its probably not as much of an issue as it is with a military gun that it work reliably without fiddling with the gas adjustment. Curious, not being confrontational. Educate me.

 

[RX-79G]

BTW, here's a list of gas port sizes used in different AR set ups. You can see that 16" carbines currently use the smallest port size:

http://www.tacticalmachining.com/learn/ar-style-rifles/ar-15-gas-port-sizes.html