AR15: Carbine Length Gas System v. Mid-Length Gas System

[Bartholomew Roberts]

I saw this technical post on AR15.com in the archives and since it described the issues around the development of the midlength gas system for the 16" barrels so well, I thought I would post it here as well:

In order to understand this, there are a few things about the functioning of the AR that have to be defined, I am away from most of my notes and stuff, so most of the figures given are from memory... but should be pretty close. For this description, the standard rifle gas system is with the gas port located at 13.0" and having a 20" barrel... the standard carbine system is with the port at 7.5" and a barrel of 14.5"

The pressures at the gas ports are: 13.5K for the rifle and 26K for the carbine -- or twice as much.

The dwell time (the time that the gas system is charged with high pressure) is determined by the amount of barrel after the gas port. These are nearly identical between the rifle and the carbine.

Pressure from the port is regulated only by the size of the gas port and the diameter of the barrel.

These two factors determine the internal bolt pressure, the maximum pressure that is obtained in the bolt carrier/piston combination -- for the rifle this pressure is about 1000psi and for the carbine it is over 1500psi, half again as much.

When the rifle is fired, primer shot sets the bullet forward until it contacts the rifling, at this point the powder charge detonates and sets the shell case fully back, binds the action and start to propel the bullet. The bullet jumps slightly again and is etched by the rifling... it stops again very briefly as the pressures build to a point for the bullet to overcome the mechanical advantage of the rifling twist and the bullet starts to spin, at this point the chamber pressure is at max, 50K plus (there are some that believe there is another, third stop the bullet makes and some testing suggest this may be true).

As the chamber pressures start to climb, the brass case expands and becomes plastic, this is essential to seal the case in the chamber -- the correct term for this is Obturation, when the case is obturated and sealed, it is stuck in the chamber, practically welded in really.

The Lock Time, or the time that the action remains locked with no attempt to start unlocking is very important... on the rifle, the lock time is about 550 microseconds, the lock time for the carbine is about 375 microseconds -- this may not seem like much, but it is much shorter of a time, also keep mind that the chamber pressures are twice as high in the carbine when the unlocking starts.

What does all of this mean? When the carbine is fired, the system attempts to unlock earlier than intended and while the case is still fully obturated... this results in the action bind delaying the unlocking and stressing the system. As the 5.56N is not drastically tapered, "squirting" is not a big problem in most guns. When the internal bolt pressures finally unlock the bolt, the velocity of the reward movement in the carbine is much higher than what the rifle was designed for, it also must start extraction of the obturated case... as you know, the AR does not have any sort of initial extraction, perhaps the single biggest shortcoming of the design. This has been known to cause ripped case heads...

At this point, as the bolt starts to unlock, it is rotated to unlock...here there is a phenomenon called extractor lift where the extractor lifts off the rim of the cartridge case -- some argue that the pressure of the extracted cartridge case keeps the case head against the bolt face, but the fact is that the extractor does float and the contact with the case rim becomes "soft". For this reason, it is much more likely that the extractor will simply pop off, rather than actually rip the case.

Balanced extractors and different designs have been developed (LMT), but the best solution to date has been stronger extractor springs and spring buffers. That about covers the FTE issues...

Back to bolt velocity. The high speed of the bolt has a couple of other detrimental effects, one of the most common is that the bolt is cycled so fast that as it returns to battery, it actually has enough force to "bounce" off of the barrel extension when closing and locking... this bounce back is very small, but can be enough to cause the weapon not to fire... this "bolt bounce" is pretty well known.

One other problem is that the bolt can cycle so fast the magazine spring can not keep up with it and the round stack is not properly aligned and forced back into place before the bolt returns to batter -- therefore there is no new cartridge picked up and the bolt closes on an empty chamber, this is what some call "ghost loading", or bolt-over-base jams... this is far worse in full auto fire as the bolt does actually move faster in full auto than semi auto; this is due to the fact that the top cartridge in the magazine does not apply force to the bottom to the bolt causing drag.

The common solution to this issue is to use a stronger recoil spring and a heavier buffer... this works, but is treating the symptom, not the problem.

PigTail and expansion chamber gas tubes attempt to fool the rifle into thinking that the gas port is, located further away that it really is, but they are not as good of a solution as actually moving the gas port out...

 

[blackhawk2000]

Gee, and I was just thinking about getting a midlength, cuz it looked "more right" than a 16" carbine.

 

[DougW]

I think this applies more to a full auto type of rifle than a semi only. I have a carbine, a mid length, and a 20" AR's. All 3 work fine.

 

[blackhawk2000]

Doug, the shorter the gas system, the more "wrong" it is. Carbine length AR15's are "violent" for lack of a better word.

 

[walking arsenal]

as you know, the AR does not have any sort of initial extraction, perhaps the single biggest shortcoming of the design. This has been known to cause ripped case heads

AFAIK, This isn't true. The bolts rotation just before it extracts the spent case is the initial extraction. The rotation breaks the case loose from the chamber walls.

The common solution to this issue is to use a stronger recoil spring and a heavier buffer... this works, but is treating the symptom, not the problem.

Equal and opposite reactions. The heavier buffer and spring would return the bolt to battery faster. This might keep the bolt from rebounding but it isnt going to help the rifle any.

I also agree this applies more to full auto guns than semi.

 

[Bartholomew Roberts]

All of these issues apply to semi-autos as well as full-autos, some of the common "reliability enhancements" you see for carbines such as stronger extractor springs, O-rings, etc. are developed as a direct result of this problem.

This isn't to say that the carbine is unreliable; but just to say that as barrels and gas systems get shorter in a direct-impingement AR, you start cutting into your reliability margin. The natural conclusion isn't that a carbine is unreliable; just that a midlength is more reliable than a carbine - and a rifle more reliable than either. Of course lots of other things play into this as well - a well made carbine with top grade parts may well outlast a parts build rifle with parts of questionable origin.

 

[DougW]

OK Blackhawk. So the carbine is more "violent". Now what? Should I stop shooting my carbine? How many catastrophic failures have occured because of the shorter gas system? Why is the US Military issuing as many M4's as it can? What is the point? If the AR is such a crappy system, why isn't there such a huge outcry from those that have to use in in harms way demanding that the military change weapons?

I am too old to join the Army. I have no illusions of being a mall ninja. I have fun with my weapons. I compete in 3 gun matches with my AR's. If one goes tits up, then I transition to my side arm.:banghead:

 

[Bartholomew Roberts]

Should I stop shooting my carbine?

Absolutely not, there are carbines out there with 20k rounds and no significant failures. So you definitely have a huge comfort zone even if the margin may not be as high as a rifle.

How many catastrophic failures have occured because of the shorter gas system?

Quite a few depending on what you define as catastrophic - bolts breaking at the cam pin hole (definitely stops the gun) and bolts snapping off locking lugs (sometimes stops the gun; but usually not) both happen at measurably higher rates in carbines.

Why is the US Military issuing as many M4's as it can?

Logistics I imagine. The increased benefits in reliability and durability probably aren't worth the cost in replacing all the existing carbines or maintaining yet another set of gas tubes and barrels in the system. Either that or the military is looking at an even larger developmental leap (XM8 or SCAR type weapons) and doesn't want to waste money on an incremental step.

What is the point? If the AR is such a crappy system, why isn't there such a huge outcry from those that have to use in in harms way demanding that the military change weapons?

Who in this thread said the AR is a crappy system? Who even implied it? I see you are local though... drop me a PM some time and we can shoot some of my midlengths side by side with your carbine and see what the buzz is about.

 

[dleong]

if a shorter gas tube raises pressures, how would an AR pistol (say, with a 10.25" barrel) fare? Are pressures kept manageable in AR pistols by the use of a more constrictive gas tube?

The reason I ask is that I am considering building an AR pistol and would like to find out more about issues affecting its reliability of operation.

 

[Bartholomew Roberts]

Direct impingement AR pistols are plagued with reliability problems compared to carbines or rifles. You are doing a major redesign of the gas system; but you are still using components designed for a 20" rifle gas system. An AR pistol will be more sensitive to ammo as well.

The good thing though is the AR is still pretty reliable. I've seen them go thousands of rounds with no stoppages, so even a 400% increase in problems may mean 1 stoppage every 1000 rounds instead of 1 every 4000.

Are pressures kept manageable in AR pistols by the use of a more constrictive gas tube?

Just the opposite actually... because the bullet leaves the barrel so quickly after passing the gas port, you have to have an even larger gas port to siphon off as much gas as possible before it all goes out the muzzle. This means that the unlock cycle happens even faster and under even more pressure. AR pistols often use fatter or pigtail gas tubes and heavier buffers and springs to reduce pressure and delay the cycle some; but it is still an issue. The major problem in a pistol is that the brass is still stuck to the wall when extraction begins which is bad; but if you delay extraction until the brass shrinks away like it would in a rifle, you won't have enough gas pressure to cycle the weapon - so you just start the cycle early and hope for the best. Most of the time it will work; but just be hard on the bolt and internals designed for much less demanding pressures. Sometimes it won't work and the gun will have a stoppage.

 

[444]

I have an AR upper with a 7.5" barrel.
I had to use every one of those tricks to get it to run.
Hard chromed extractor (Les Baer), Wolff extra power extractor spring with the black insert, "O" ring, and one of those buffers designed to decrease the cyclic rate of a machine gun: the name escapes me at the moment.
I never tried a heaver buffer spring.

 

[Crosshair]

From what I understand, the AR does have a little primary extraction, but not much. We also have to remember that the AK has no primary extraction as well. So it is not the lack of PE that is the problem, but other factors.

The AK uses a very large and powerfull extractor to grip the case and pull it out. The AR extractor is small compared to the AK extractor. The AK uses the massive bolt carrier and bolt to "store" the energy given to it by the gas pressure while the AR relies more on the gas volume given to it by the gas tube.

What going to the Mid-Length gas system does is change the gas impulse from a sharp punch to a more gently push resulting into less wear and greater reliability. (Not to mention a slower cyclic rate.) The AK has a litle more leeway in regard to gas impulse since the mass of the bolt carrier tends to smooth it out.

The AR is much easier to fine tune than the AK, but it is also much easier to screw up if you don't do it right. The Mid-Length gas system is a step in the right direction.

 

[Still 2 Many Choices!?]

crosshair,

When comparing extraction in the AR and AK, you can't forget the sloped case of the 7.62X39 cartridge also. It was mentioned in the article, and really does make a big difference. Especially when added to the other factors you already mentioned .

 

[chopinbloc]

the shorter the gas system, the more "wrong" it is. Carbine length AR15's are "violent" for lack of a better word.

one thing that makes this issue worse for us lowly peasants is the legal minimum of 16" for the barrel. the carbine lenght gas system was - as is mentioned above - designed for a 14.5" barrel. when you add another inch and a half of barrel you get a longer dwell time than either the rifle or the carbine is supposed to have. as i understand it, a midlength gas system is better tuned for a 16" barrel.

 

[Jim K]

Permit me to take some issue with the following:

"When the rifle is fired, primer shot sets the bullet forward until it contacts the rifling, at this point the powder charge detonates and sets the shell case fully back, binds the action and start to propel the bullet. The bullet jumps slightly again and is etched by the rifling... it stops again very briefly as the pressures build to a point for the bullet to overcome the mechanical advantage of the rifling twist and the bullet starts to spin, at this point the chamber pressure is at max, 50K plus (there are some that believe there is another, third stop the bullet makes and some testing suggest this may be true)."

The primer force does not start the bullet forward to contact the rifling; it ignites the powder and sets the primer back. The powder charge does not detonate (if it does, the rifle blows up), it burns rapidly. That pressure causes the case walls to expand to grip the walls of the chamber. It also pushes the base of the case back against the bolt. (It is this that causes head separation if headspace is excessive.)

The inertia of the bullet keeps it in place while the expanding gas pushes out the neck of the case, then rushes out around the bullet. (It is this hot jet of flame in the narrow space between the bullet and the barrel that causes throat erosion. Throat erosion stops at the point the bullet seals the barrel.)

Once the bullet starts moving, it doesn't stop at the rifling, nor does the rifling hold it. It first tries to move straight ahead, but if the throat is correct, it will fit into the rifling and be spun by it. Note that the bullet is not expanded into the rifling - it is groove diameter, so the lands are forced into it. If the throat allows the bullet to tip or expand prematurely, it enters the rifling unevenly and is distorted so it becomes inaccurate. (That is why excessive throat erosion causes inaccuracy.)

Nor does peak pressure occur as the bullet enters the rifling; it occurs when the bullet is about 1-2 inches into the barrel, which is why the rear "swell" of the barrel extends further than the end of the cartridge case.

 

[blackhawk2000]

OK Blackhawk. So the carbine is more "violent". Now what? Should I stop shooting my carbine? How many catastrophic failures have occured because of the shorter gas system? Why is the US Military issuing as many M4's as it can? What is the point? If the AR is such a crappy system, why isn't there such a huge outcry from those that have to use in in harms way demanding that the military change weapons?

I am too old to join the Army. I have no illusions of being a mall ninja. I have fun with my weapons. I compete in 3 gun matches with my AR's. If one goes tits up, then I transition to my side arm.

Lighten up Francis.

I don't know what set you off on that tangent, but you don't know jack about what I think of the AR, or how I use mine, or if I even own one.


The others have answered far better than I could, as to what I meant by "violent".

 

[Onslaught]

OK Blackhawk. So the carbine is more "violent". Now what? Should I stop shooting my carbine? How many catastrophic failures have occured because of the shorter gas system? Why is the US Military issuing as many M4's as it can? What is the point? If the AR is such a crappy system, why isn't there such a huge outcry from those that have to use in in harms way demanding that the military change weapons?

I am too old to join the Army. I have no illusions of being a mall ninja. I have fun with my weapons. I compete in 3 gun matches with my AR's. If one goes tits up, then I transition to my side arm.

I didn't get where Blackhawk was suggesting any such thing. Being the owner of several AR type weapons, I also did NOT take offense to his comments or feel the need to get upset about them.

I have three 16" or less ARs... One carbine gas system and two mid-lengths. I like both. I shoot both. But that doesn't change the fact that the carbine has more and sharper recoil than the mid-length systems.

None is painful, but to a 10 year old child (my son, who's 14 now) it was the difference between shooting and not shooting. He only shot my Bushmaster with a fluted HBAR (not even the superlight, so it wasn't the weight) three rounds then said it "hurt", but when I bought a government profile mid-length upper from Rock River just two months later, he tried again and didn't want to stop.

 

[grizz]

I wish I would have known this before I shelled out $1k on a new RRA carbine.

So, If I only have funds for 1 AR, and I want the most reliable weapon possible (in the AR format) should I sell my practically new carbine and buy a 20 inch rifle?

If I like the 16 inch barrel, should I sell my carbine and go for a midlength?

Or, should I just keep the carbine, since I'd probably loose $150 off what I paid for it by selling it used and save up for a different AR later?

What would you do?

Also, does a longer barrel length (like a 24 inch varminter) negatively effect the operation of the gas system?

 

[soul_rapier]

The way i look at it keep the ar and just save up for another if thats what you want. i have 2 ar's and i like the m4 type better then my full size

 

[Jeff White]

I have not seen any of the reliability problems the mid length is supposed to fix on any properly built semi-auto carbine. I've only seen those problems in full auto. When the stronger extractor spring with the black insert first came out, it was only required to be installed in the M4A1. M4s which are 3 round burst were considered serviceable without it. The new spring was only standardized throughout the M4/M16 family of weapons so the supply system didn't have to stock both.

I have a carbine I built out of all Colt parts on a Bushmaster lower that had a blue insert extractor spring until just recently. It's been used just as hard as my Colt 6920 LE Carbine (which came with the new extractor spring) and has yet to have as much as a burble.

Where you end up with problems is with the higher cyclic rate. I converted 3 M16A1s the sheriff's dept received through the 1033 program to M4 configuration for the tac team. The all functioned just fine in semi. They had to be upgraded with "H" buffers to work in auto. I also upgraded the extractor springs at the same time. No problems since.

My personal opinion is that people spend too much money on parts for full auto weapons to put on their semis. If you buy a carbine from a teir one manufacturer, i.e. Colt, LMT, Noveske, Bravo Company or build a carbine from parts that are in spec you won't have problems like you do with the lower tier manufacturers. My experience with semi autos that don't run has always been traced to unstaked or improperly staked carrier keys, the wrong size roll pin in the front sight base holding the gas tube in place, out of spec gas ports on that bargain barrel or that were made out of spec by some shadetree gunsmith who thought that opening it up would fix an unstaked carrier key. Gas tubes that were plugged by people trying to clean them, cheap plastic not in spec buffers, out of spec buffer springs. The number of guys I know who've dropped a lot of money on reliability enhancements for full auto weapons trying to fix those problems with their bargain basement carbines has always amazed me.

Jeff

 

[rallyhound]

So then, which gas system is on the dissapator model?
Looks like a 16 inch barrel with the rifle gas system to me.

 

[Coronach]

This Dissy has a carbine length gas system under the handguards. The front "gas block" is merely there for the front sight.

Mike

 

[Bartholomew Roberts]

So, If I only have funds for 1 AR, and I want the most reliable weapon possible (in the AR format) should I sell my practically new carbine and buy a 20 inch rifle? If I like the 16 inch barrel, should I sell my carbine and go for a midlength? Or, should I just keep the carbine, since I'd probably loose $150 off what I paid for it by selling it used and save up for a different AR later?

What would you do?

If it were me, I would just keep the carbine and shoot it until the barrel was shot out. When you replace the barrel, you can also replace the gas system with whatever happens to be best at that point in time.

In the meantime, you can upgrade the extractor insert, extractor spring, and buffer (black insert, SAW Heavy-duty extractor spring, H-buffer) if you are concerned about reliability. Those three changes fix most of the problems caused by the shorter gas system (and they are stock in a Colt carbine as Jeff mentions, though they are rarely part of aftermarket carbines such as RRA).

However, I generally recommend following the "If it ain't broke, don't fix it" principle. A lot of so-called "reliability enhancements" only enhance reliability within a very specific set of shooting conditions and when you start applying them without understanding how they work in the entire system, you actually degrade reliability. Replacing the extractor insert and extractor spring is almost always a "safe" upgrade that doesn't harm anything. Depending on the rifle though, adding an H-buffer may not be an improvement to reliability. In an RRA, it is probably a safe but unnecessary option.

Finally, keep in mind we are talking about extremes here. The difference between lock time in a rifle and a carbine is 175 microseconds. For reference, a microsecond is 1/millionth of a second. The difference in lock time between a carbine and midlength is even smaller. It does make a difference in operating pressures which should theoretically make a difference in reliability and long-term use; but we aren't talking a dramatic difference. Individual deviation in assembly accounts for a much larger difference in reliability than the difference in gas system. The big thing that is driving the use of midlengths isn't reliability as much as the smoother recoil impulse which is very popular for 3-gun and competition style shooting.

So then, which gas system is on the dissapator model?
Looks like a 16 inch barrel with the rifle gas system to me.

Some Dissipators use a carbine length gas system with a second gas block underneath the handguard (Bushmaster). Some use a midlength gas system (CMMG). Some use a rifle length gas system. The rifle length gas system is generally the least desirable with a 16" barrel because you run into the same problem you do with short-barrelled ARs. There is not enough barrel after the gas port for the system to draw off enough gas before the bullet exits the barrel. Personally, I would not use a rifle length gas system on any barrel shorter than 17"; but some people have been able to make them work on 16" barrels.

 

[taliv]

grizz, unless you're just overly attached to having all the parts in your rifle made by or at least assembled by the same company (in theory), my recommendation would be to read this thread for some ideas
http://thehighroad.org/showthread.php?t=233214

and consider rolling your own upper. instead of selling the whole RRA, just part out the barrel assembly, and purchase a new barrel, gas-block and tube. i'd also recommend the YHM specter forearm in rifle length, with a low-profile gas block that fits underneath it.

otoh, much as i hate to agree with jeff , there's really no need to worry about reliability just because it's a carbine. spend your $ on an extra 1000 rnds of ammo and let us know how reliable it was.

 

[Kaylee]

wow -- thanks for the education all!

Happy day. Learned something new.