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


PDA






Bartholomew Roberts
May 10, 2006, 01:39 AM
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...

If you enjoyed reading about "AR15: Carbine Length Gas System v. Mid-Length Gas System" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!
blackhawk2000
May 10, 2006, 09:51 PM
Gee, and I was just thinking about getting a midlength, cuz it looked "more right" than a 16" carbine.

DougW
May 10, 2006, 09:59 PM
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.:D

blackhawk2000
May 10, 2006, 10:23 PM
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
May 10, 2006, 10:57 PM
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
May 10, 2006, 11:34 PM
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
May 11, 2006, 10:53 AM
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
May 11, 2006, 11:50 AM
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
May 11, 2006, 12:19 PM
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
May 11, 2006, 12:26 PM
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
May 11, 2006, 02:06 PM
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
May 11, 2006, 05:11 PM
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!?
May 11, 2006, 05:27 PM
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:o .

chopinbloc
May 11, 2006, 05:48 PM
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
May 11, 2006, 07:50 PM
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
May 12, 2006, 12:34 AM
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
May 12, 2006, 11:47 AM
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
November 25, 2006, 02:24 AM
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
November 25, 2006, 02:50 AM
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
November 25, 2006, 06:40 AM
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
November 25, 2006, 12:01 PM
So then, which gas system is on the dissapator model?
Looks like a 16 inch barrel with the rifle gas system to me.

Coronach
November 25, 2006, 12:35 PM
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
November 25, 2006, 12:39 PM
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
November 25, 2006, 12:47 PM
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 :neener: , 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
November 25, 2006, 01:42 PM
wow -- thanks for the education all!

Happy day. Learned something new. :p

grizz
November 25, 2006, 06:04 PM
Well, I guess I'll probably just keep it until it gives me problems, then decide what to do. I've only put about 200 rounds through it and it's been flawless so far.

Thanks everyone.

Coronach
November 25, 2006, 06:51 PM
I think you're worrying far too much about it, Grizz. There is nothing wrong with the carbine.

Is its action cycle more violent and rough on the gun than a rifle's? Yes. Is it more violent and rough on the gun than a 16" middy? Yes. Is it destined to shake the poor gun apart in so short a period of time that you wasted your money? Absolutely not. These guns get used full auto in the Sandbox. They work.

The differences being discussed are pretty small things. Even the proponents of the midlength gas system are readily admitting that other factors, like quality of parts and quality of assembly, will have a far greater impact on the service life of the weapon than the differences between a midlength and carbine-length gas system on an M4 style weapon. The phrase "all else being equal" applies here, and very very rarely is everything else equal.

Even if it is, the differences are pretty small. If you have put 200 rounds through a rifle that should be able to go 20,000 before anything breaks- and that parts failure will probably cost a couple of bucks to fix. You're just 1 percent of the way there. By comparison, you're probably 30 percent, or more, of the way through your life.

Bigger things to worry about, mate.

Mike

worker
November 26, 2006, 07:39 PM
very educational thread

thank you for sharing such valuable info,

I wanted to ask if .308 carabines such as

http://www.dpmsinc.com/firearms/308/ap4.aspx

benefit from the fact that it is a .308 round and therefore
shorter barrel is still enough to keep enough pressure in the
system. And what type of the gas system does this DPMS carbine has, does any one know?


thanks in advance

Bartholomew Roberts
November 26, 2006, 11:19 PM
worker, a short barrel direct impingement rifle is going to have higher pressures than a long barrel direct impingement rifle generally. I do not know whether the DPMS was designed with the pressures of a short gas system or long gas system in mind. If it was designed with a short gas system in mind, then the components will be better built to withstand the higher pressures and you can change to a longer gas system by lightening the buffer. If designed with the long gas system in mind, there will be more stress on internal parts in the short system and you'll need a heavier buffer and beefed up extraction when using a short barrel.

rbernie
November 26, 2006, 11:24 PM
A 308 has a completely different expansion ratio (roughly, bore volume to powder volume) than 223. A carbine length gas system on a 308 will therefore not necessarily have the same issues as will the same system in 223.

For example, I've tried, via two different builds, to get a 7.62x39 rifle length system to work. There is simply not enough gas from that small powder charge (relative to bore volume) to make it work reliably. But I can get 7.62x39 to work like a superflychamp in a 16" barrel with a carbine-length gas system.

K.L. Davis
April 17, 2008, 02:17 AM
It is a small world... I wrote what was quoted in the original post some time back. Originally there was a "disclaimer" at the beginning that said I was writing it while deployed and going only from memory -- I get a lot of grief about the powder "detonating" thing, of course that is not right.

I have to stick with what I know though... several tests conducted do show that the bullet does stop a couple of times during the initial travel, granted it is such a short period that it is nearly unmeasurable...

AR15barrels
July 22, 2008, 03:13 AM
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.

The AR has no primary extraction.
The front face of the bolt lugs are all flat and the breech face of the barrel is flat too.

http://ar15barrels.com/tech/ar15bolt-extension.gif

As for the bolt carrier being pushed back by gas, that's not true.
Once the bolt carrier moves about 1/8", it opens the exhaust ports and vents out any pressure in the carrier.
The kinetic energy stored in the carrier is what keeps the carrier moving rearward, not the actual gas that may still be in the gas tube and gas key.

wacki
November 9, 2008, 07:54 PM
rolling your own upper

What does that mean? rolling?

Gord
November 9, 2008, 08:08 PM
Building.

You know, "roll your own."

H2O MAN
November 9, 2008, 08:33 PM
All of my research indicates that the combination of a mid-length
gas system and a 16" barrel is ideal for a semi-automatic .223 AR.

highorder
November 9, 2008, 10:44 PM
Correct me if I'm wrong, but the carbine gas system was designed for a 10" or 11" barrel.

The 14.5" M4 profile came 20 years later.

45B@cav
November 10, 2008, 02:50 PM
I don't mean to get of subject but I would not do the whole extracter upgrade with the O-ring thing. On my last deployment I had 2 cotractors carbines come in with bolts broken off at the extracter pin. Both had O-ring around the spring. I replaced them with the upgraded springs and warned against doing it again the contractor didn't have anymore problems after the fix. Has anyone else heard of this happening?

Lewisusa1
February 8, 2009, 07:37 AM
Could someone please tell me the actual lengths involved with the following?:

1. Carbine length: Barrel length and gas port location.

2. Mid length: barrel length and gas port length.

3. Std/Rifle? length: barrel length and gas port length.

I have a Bushmaster Dissy, which I understand has a carbine length gas system on a 16" barrel. Is this considered a "mid length" setup?

I am in the process of building 2 more AR's and 1 has a 20" barrel and one has a 16" barrel.

I am speculating that the 16" would be a mid length and the 20" inch would be a std or Rifle? length. So would a carbine be 11.5" or anything less then 16"?

Thanks for the help.

Chris

P.S. feel free to respond at Lewisusa1@comcast.net as I am not sure if I will be able to find this thread again as I am new to this site and it appears huge.

taliv
February 8, 2009, 10:40 AM
lewis, "mid-length" and "carbine length" actually refer to the distance between the gas port and the chamber, not the length of the overall barrel. so you can have a carbine-length gas system on a 10.5" barrel, or a 12" barrel or a 16" barrel or an 18" barrel etc.
you can have a mid-length gas on a 12" barrel or a 16" barrel or a 18" barrel.

briansmithwins
February 8, 2009, 12:08 PM
I've always learned 'primary extraction' as the bolt rotating before beginning to remove the cartridge from the chamber. Both the AK and AR do this, as opposed to the SKS or HK roller delayed system where the case is removed from the chamber without rotating.

The moving mass in the AR and AK is very close so you might as well give eup on the whole 'massive bolt carrier' thing. BSW

AK bolt and bolt carrier = 508 grams
http://i2.photobucket.com/albums/y3/briansmithwins/AKrecipparts.jpg

AR bolt, bolt carrier, and buffer = 468 grams
http://i2.photobucket.com/albums/y3/briansmithwins/ARrecipparts.jpg

Dr. Tad Hussein Winslow
May 9, 2009, 07:01 PM
OK, so we learned on page 1 that the gas port is at:

13.2" on a rifle system,

7.5" on a carbine system

Where is the gas port on a mid-length system - at what distance from the breech? What's the longest and shortest barrel found with mid-length systems - i.e. what's the range of possible working barrel lengths?

The original post in this 2006 thread, though informative, purports to compare a carbine system to a mid-length system, but then goes on in the OP to discuss the carbine system and the rifle system, not the middy system.

Is it 10.0", 10.5", or what?

Thanks.

Dr. Tad Hussein Winslow
May 9, 2009, 07:24 PM
Found it, 9.8"

http://www.ar15.com/forums/topic.html?b=3&f=12&t=384703

Bartholomew Roberts
May 11, 2009, 11:33 AM
Correct me if I'm wrong, but the carbine gas system was designed for a 10" or 11" barrel.

The carbine gas system was designed for a 10.5" barrel. However, they discovered that the weapon would not function reliably due to the extremely short dwell time. This was one of the purposes of the moderator on the XM177 - it not only reduced the report to the same as a 20" barrel, the buildup of backpressure helped functioning. However, they still had problems and the barrel was lengthed to 11.5" for reliable functioning.

I don't know for sure how they arrived at 14.5" for the M4; but it turns out that the extra difference in barrel length increases the dwell time to about the same as what it is in a rifle or midlength.

I don't mean to get of subject but I would not do the whole extracter upgrade with the O-ring thing. On my last deployment I had 2 cotractors carbines come in with bolts broken off at the extracter pin. Both had O-ring around the spring. I replaced them with the upgraded springs and warned against doing it again the contractor didn't have anymore problems after the fix. Has anyone else heard of this happening?

It is common for those bolt lugs to break. For example, look at this March 15, 2002 Technical Note from Armalite describing just that issue (http://www.armalite.com/images/Tech%20Notes%5CTech%20Note%2050,%20The%20Cycle%20of%20Operation%20as%20a%20Guide%20to%20Diagnos%E2%80%A6.pdf). For one; both bolt lugs are undercut in order to make room for the extractor, so they are weaker than the other bolt lugs. Two, a carbine (M4) gas system puts about 50% more load on the bolt (which was designed for a 20" gas system).

I've got a PDF or Powerpoint somewhere showing the loads on the M16 bolt during operation. If I remember correctly (take this with a grain of salt as that is a big if in this case), the absence of a lug on the extractor also places a higher load on the adjacent bolt lugs which is why these typically fail first.

Personally, I don't think the O-ring has anything to do with the breakage in your case. What hypothesis did you have in mind for how the O-ring contributed to the breakage?

What's the longest and shortest barrel found with mid-length systems - i.e. what's the range of possible working barrel lengths?

The shortest that I would go on a midlength is 14.5" and even then I think you are pushing it. The longest I would go is 16.5". You can go longer than that; but after 17", you can reliably use a rifle-length gas system so you are actually increasing the dwell time beyond the original design parameters and creating an effect similar to a carbine length gas system with a 16" barrel or suppressor.

Zak Smith
May 11, 2009, 12:55 PM
Noveske makes a 14.5" "Afghan" barrel which has a MLGS. I had one and it ran perfectly. His 12.5" uses a carbine gas system. My 12" has a CLGS. I believe it's better to use the longest gas system which will operate reliably given the barrel length. Like Bartholomew Roberts said, a RLGS works fine on 17" barrels (my two MSTN 3-Gun rifles are set up that way). I know a couple people who cut down 20" rifles (RLGS) to 16" and they still worked OK, but I think that's not going to work in all cases.

Coronach
May 11, 2009, 06:59 PM
The moving mass in the AR and AK is very close so you might as well give eup on the whole 'massive bolt carrier' thing. BSWOff the cuff, I would say that the difference is not as simple as reciprocating mass. That's a component of it, of course, but qually important would be things like where is the reciprocating mass? In the AR it is all in direct line with the bore and the shoulder. In the AK it sits above the bore, which in turn sits above the shoulder.

Mike

jungle
May 11, 2009, 10:32 PM
"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."
-------------------------------------------------------------------------------------------------------


The writer sounds astute until this point. Unlocking doesn't really start until the bullet has left the barrel and pressures are way down, near zero. As AR15 Barrels pointed out, after a short travel the gas driving the bolt is vented. In effect the system is self-regulating as far as pressure in the bolt. The carbine bolt may start faster, but it starts venting faster too.

Colt defense lists the cyclic rate for the M16 and M4 as the same, this may be corporate oversight, but I have not noted any great difference in any publication that the cyclic rate of these two weapons differs to any great degree.

If the gas was having a 50% greater impact on bolt carrier speed, the cyclic rate would surely be much higher on the M4.


13.2 , 9.8 and 7.5 are the length of the respective gas systems. Subtract that from the length of the barrel and you have an idea of the time the gas is acting on the system with a bullet in the barrel, it is longer in the case of an M16 and shorter with an M4 with a mid-length system about the same as an m4 with a carbine system. Lower pressure over longer duration vs higher pressure over shorter duration. It works out to be not too different.

Gas port size is also a huge factor in the total equation. It will have a strong influence on the total volume of gas delivered to the system.

Zak Smith
May 11, 2009, 10:38 PM
Here is ArmaLite's data on pressure vs. time
http://www.armalite.com/images/Tech%20Notes%5CTech%20Note%2048,%20Barrel%20Design,%20Heat,%20and%20Reliability,%20030824%E2%80%A6.pdf

jungle
May 11, 2009, 10:46 PM
As discussed above, the distance from the chamber to the gas port is important. So too is the length of the
barrel past the gas port.
That’s because the bullet serves as a plug to keep the gas pressure trapped in the barrel so that some of it
can pass into the gas tube and back to the carrier. If the length of barrel beyond the gas port is too short, so
is the “dwell” of the plug in the barrel. The gas pulse supplied to the carrier can be too short to deliver all of
the energy that the carrier group needs. Too long a section of barrel beyond the gas port can cause too
long a gas pulse.
Armalite

It is also worth noting the actual size of the gas port drilled in the barrel can have a large effect too and these can vary with configuration.

Bartholomew Roberts
May 11, 2009, 11:56 PM
The writer sounds astute until this point. Unlocking doesn't really start until the bullet has left the barrel and pressures are way down, near zero.

OK... so in a rifle length gas system, there is 6.8" of barrel after the gas port and it takes 550 microseconds to unlock. So the bullet (which is travelling slightly faster in the 20") has left the barrel by the time the unlock cycle starts.

Now, if we lengthen the amount of barrel past the gas port so that there is now 8.5" of barrel past the gas port (as there would be on a 16" barrel with a carbine gas system) AND we have reduced the lock time to 375 microseconds as well as reduced the velocity of the bullet, then has the bullet exited the barrel as you contend?

The carbine bolt may start faster, but it starts venting faster too.

The extra load on the carbine bolt isn't from the bolt travelling faster - it is from the carbine trying to extract and eject earlier in the cycle while the case is still obturated.

If the gas was having a 50% greater impact on bolt carrier speed, the cyclic rate would surely be much higher on the M4.

There is a 50% greater load on the M4 bolt. That doesn't mean the bolt is travelling 50% faster.

jungle
May 12, 2009, 12:05 AM
His figures don't make sense, is there a source? Do you contend as the writer does, that the barrel/chamber of the carbine is under twice the pressure of the rifle at unlock? In fact the pressure at unlock would be the same.

Any comments on the differece in gas port sizes between the different configurations?

Why no inrease in cyclic rate if the carbine bolt carrier is moving faster? The shorter lock or dwell time cited by the author MUST mean he thinks the carrier is moving faster.


"Now, if we lengthen the amount of barrel past the gas port so that there is now 8.5" of barrel past the gas port (as there would be on a 16" barrel with a carbine gas system) AND we have reduced the lock time to 375 microseconds as well as reduced the velocity of the bullet, then has the bullet exited the barrel as you contend?"

You bet it has, otherwise you would get a face full of gas and metal. At the very least, high residual chamber pressure in any configuration would get you a lot of case heads torn by the extractor because the brass won't give up it's grip on the chamber.

Take a look at the cam slot on the carrier, it moves at least an eigth of an inch before any unlock takes place, a lifetime at the speeds we are talking about.

cameron.personal
May 12, 2009, 12:09 AM
Every time I shoot my carbine length gas 16" Colt and my SBR 10.5" LMT I am just completely astounded, even flabbergasted, maybe even amazed at the shear miracle that the actually feed, fire and extract round after round with boring regularity....

Oh, I have a midlength 16" too, it runs like the others...


:rolleyes:

jungle
May 12, 2009, 12:25 AM
I suspect the details have been worked out at length to give suitable function in many configurations.:)



"Through exhaustive testing and end user operator input, SOCOM/Crane-NSWC created a kit that helps to remedy extraction problems experienced by end users of M4A1 carbines. Most extraction problems have been traced back to the doubling of peak gas port pressure and 20% shorter dwell time on the carbine length gas system over the standard full length gas system on the M16A1/A2 rifles. This causes faster bolt speeds, and more violent extraction. The BHI M4 SOPMOD kit includes all the upgrades of the Crane version with the addition of a rate reducing heavy buffer, and an extreme duty Sprinco buffer spring."



Faster initial bolt carrier movement, not residual pressure is the culprit.

Bartholomew Roberts
May 12, 2009, 04:22 PM
Faster initial bolt carrier movement, not residual pressure is the culprit.

How do you separate the two? The reason faster initial bolt carrier movement is a problem is because the brass is still expanded against the chamber wall due to residual chamber pressure and the extractor has to work harder to remove the casing. That is the whole purpose of the kit you mention - to add spring tension to the extractor so it will not slip off the rim even though the brass is still "sticky" due to the higher residual pressure in the carbine.

Check out this link: http://www.ar15barrels.com/prod/operation.shtml

It explains the cycle well and has pictures showing what happens to the brass when the extraction cycle starts too early due to too much gas.

jungle
May 12, 2009, 08:42 PM
Check out this link: http://www.ar15barrels.com/prod/operation.shtml


Good link and interesting that his dwell time figures are far different than those in the opening article, his data show that a midlength gas system has some effect on dwell time, but not a huge effect and certainly not the 50% difference that the original article implied.

Bottom line: gas port size, location and other factors can be altered to produce reliable weapons in many lengths. The difference between a carbine with a mid length or carbine length gas system is of mostly academic interest. It may mean bolt replacement at 20k vs 25k and has little real meaning to the average civil or military user.

In any of these arangements the bullet has left the barrel prior to unlocking, whether that allows adequate time for springback of brass, or the higher speed of initial extraction actually creates a problem for most users is debatable.
Carbine length gas systems have proven quite reliable over the long run, I think barrels and bolts will wear out on any configuration before the differences are very noticable.

RP88
May 12, 2009, 09:30 PM
wait...is midlength bad or something? I'm 300 rounds into my CMMG 16" midlength and it has only messed up once, and it was a faulty extraction issue that I would attribute to the green followers...

Jeff White
May 13, 2009, 12:10 AM
I'm 300 rounds into my CMMG 16" midlength and it has only messed up once, and it was a faulty extraction issue that I would attribute to the green followers..

How would the followers in the magazine cause faulty extraction?

Coronach
May 13, 2009, 12:16 AM
wait...is midlength bad or something?No, midlength is good. The debate is about how much better/more reliable it is than carbine-length gas. IOW, is the difference anything worth worrying about, or not? But no, nothing wrong with the middy.I'm 300 rounds into my CMMG 16" midlength and it has only messed up once, and it was a faulty extraction issue that I would attribute to the green followers...I think you're using the wrong term. Extraction refers to the action of pulling the fired brass out of the chamber, immediatly prior to ejection. I think you're talking about feeding. Think "feeding into the chamber" instead of "extracting from the magazine".

Mike

RP88
May 14, 2009, 08:22 PM
no, I'm blaming the magazine, lol. The empty casing re-seated itself half-way into the mag, with the follower nose-diving into the mag a bit:uhoh:

still trying to wonder how that happened... Maybe it was magic

Jeff White
May 14, 2009, 08:44 PM
no, I'm blaming the magazine, lol. The empty casing re-seated itself half-way into the mag, with the follower nose-diving into the mag a bit.

Carbine or rifle? Does it just do it with one magazine?

07pats
June 22, 2010, 03:55 PM
So a Daniel Defense DDM4V3 is not a good idea - these are supposedly SUPERB rifles are you people saying differently???

Zak Smith
June 22, 2010, 03:59 PM
The DDM4V3 is midlength rifle, so the main thesis of this thread would support that it is better than the original DDM4. Your question makes no sense and nobody else said anything about DD rifles or the DDM4V3 in this thread other than you.

I am locking this thread since it was resurrected without sufficient reason. If you have a coherent question, post a new thread.

If you enjoyed reading about "AR15: Carbine Length Gas System v. Mid-Length Gas System" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!