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

[cameron.personal]

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...

 

[jungle]

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]

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]

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]

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]

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]

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]

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

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

 

[Jeff White]

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]

So a Daniel Defense DDM4V3 is not a good idea - these are supposedly SUPERB rifles are you people saying differently???

 

[Zak Smith]

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.