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

[grizz]

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]

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]

.308 dpms

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]

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]

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]

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]

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.



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]

rolling your own upper

What does that mean? rolling?

 

[Gord]

Building.

You know, "roll your own."

 

[H2O MAN]

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]

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]

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]

Mid length vs. carbine length vs. std length

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 [email protected] 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]

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]

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


AR bolt, bolt carrier, and buffer = 468 grams

 

[Dr. Tad Hussein Winslow]

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]

Found it, 9.8"

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

 

[Bartholomew Roberts]

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. 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]

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]

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

Off 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]

"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."
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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]

Here is ArmaLite's data on pressure vs. time
http://www.armalite.com/images/Tech...el Design, Heat, and Reliability, 030824….pdf

 

[jungle]

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]

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]

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.