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Posted: 1/26/2006 6:46:06 PM EDT
Why is the full size AR-15s gas system supposed to be more reliable than the shorter M4s?
Link Posted: 1/26/2006 7:04:14 PM EDT

My understanding is that the pressure "spike" (i.e the maximum pressure that builds up in the gas system) is higher in the shorter systems, which leads to a harsher recoil, more wear and tear - and sometimes needs to be tweaked with a heavier buffer or stronger spring.

Personally, I've NEVER had reliability problems with a 16" AR (haven't really had any reliability problems with even shorter ones either, if using good ammo) - so for me, the difference is only that the longer systems seems a little smoother.

Link Posted: 1/26/2006 7:05:19 PM EDT
More time and volume for the gas. For auto loaders slower often equals more reliable. That and it has been around longer so any issues were worked out long ago. Any one else? The 20"ers do seem to be more relable.
Link Posted: 1/26/2006 7:09:00 PM EDT
[Last Edit: 1/26/2006 7:10:44 PM EDT by SamColt]
is it really the fact that the 20in rifle gas system is more reliable than the carbine? Or is it more about the distance from the gas port to the muzzle that makes the difference? ie. the dissipator (sp?) wouldn't be nearly as reliable if the gas port were truly at the FSB - an inch or 2 from the muzzle as opposed to where it's located - under the full length hand guards?

Just guessing - I have never heard that a 16in carbine is less "reliable" than a 20in rifle.

Just thinking out loud here - I'll quit rambling and wait for the AR techs to arrive.
Link Posted: 1/26/2006 7:10:47 PM EDT
[Last Edit: 1/26/2006 7:20:12 PM EDT by Gunzilla]
It has to do with the pressure at the port location, dwell time and lock time... the bottom line is that internal bolt pressure and the lock time -- carbine gas systems have internal bolt pressure more than 50% higher than rifle length systems and the lock time is much longer shorter, causing the rifle to unlock when the chamber pressures are high


the dissipator (sp?) wouldn't be nearly as reliable if the gas port were truly at the FSB - an inch or 2 from the muzzle as opposed to where it's located


Not enough dwell time to function reliably without opening the gas port up... a lot.

edited to fix a bone head error, I was thinking rilfe and talking about carbine
Link Posted: 1/26/2006 7:13:10 PM EDT

Originally Posted By Gunzilla:
It has to do with the pressure at the port location, dwell time and lock time... the bottom line is that internal bolt pressure and the lock time -- carbine gas systems have internal bolt pressure more than 50% higher than rifle length systems and the lock time is much longer, allowing the rifle to unlock when the chamber pressures have dropped...



ok - so it's the distance from the bolt face to the gas port as opposed to the distance from the gas port to the muzzle that makes it less "reliable"?
Link Posted: 1/26/2006 7:16:58 PM EDT
it is a combination of all factors, the maximum available pressure is determined by the location of the gas port (closer to the chamber is higher), the size of the gas port and the diameter of the barrel.

The dwell time, is the time that pressure is available to the bolt and kept high, this is dtermined by the length of barrel after the gas port...

Reilability depends on a properly regulated gas system, the weight of the bolt/buffer mass and the action spring... those are the big things anyway.
Link Posted: 1/26/2006 8:50:33 PM EDT
[Last Edit: 1/26/2006 8:51:15 PM EDT by ROMAD-556]
Best link, read it and know it.

Link Posted: 1/27/2006 2:01:46 AM EDT
Thanks for the link. I'm gonna read it now.
Link Posted: 1/27/2006 2:57:45 AM EDT
Great thread and don't mean to hijack it. But after reading the info wondering if a
"pigtail" as an option for correct pressures and increased reliability if 16" or shorter system is really problematic.
Link Posted: 1/27/2006 4:28:05 AM EDT
Ok from reading that link above...... I get that my RRA CAR4 with the mid-length handguards may be a little less sensitive to ammo and pressure differences than the normal short length version?

R
Link Posted: 1/27/2006 4:49:24 AM EDT

Originally Posted By armaliteuser:
Great thread and don't mean to hijack it. But after reading the info wondering if a
"pigtail" as an option for correct pressures and increased reliability if 16" or shorter system is really problematic.



Pigtails will work, if you do not want to change your barrel.
Link Posted: 1/27/2006 5:11:51 AM EDT

Originally Posted By armaliteuser:
Great thread and don't mean to hijack it. But after reading the info wondering if a
"pigtail" as an option for correct pressures and increased reliability if 16" or shorter system is really problematic.



Pig tails are a band-aid fix that treats the symptom, not the problem...

They also have a rather spotty record due to alignment issues... the tube gets a little longer as it gets hot (naturally), so you can imagine what happens to the alignment of the pigtail as it gets hot -- as it starts to drag, the excessive flex that is inherent in the design makes things worse.
Link Posted: 1/27/2006 12:50:37 PM EDT
Link Posted: 1/27/2006 3:16:06 PM EDT
Gas port pressure for a 20" barrel is appx 13,000 psi.

For a 16" middy, appx 18,000 psi.

For a 16" CAR barrel, appx 26,000 psi.

But the port size is different for each of these to compensate.

The 20" has a longer, lower pressure pulse. The 16" CAR has a shorter, high pressure pulse. The middy in between.

There is a noticable difference between the three. The Mid Length feels much more like a 20" rifle, smoother.
Link Posted: 1/27/2006 3:23:32 PM EDT
Note in that Armalite Tech Note

"At a temperature of approximately 1100 degrees, however, the structure of this alloy undergoes a permanent transformation that substantially, and permanently, alters it."

Remember that one of the methods suggested by BATFE to permanently attach a flash hider or comp is to silver solder using a solder that melts at 1100 degrees.
Link Posted: 1/27/2006 3:30:42 PM EDT
The length to the gas tube is important, true enough. What really matters though is the length of time (also volume and pressure of gas) that is transferred to the carrier group during the dwell.

What becomes most noticeable is that the dwell time is reduced in shorter barrels as the distance from port to muzzle is shorter. The gas volume is increased to gain reliable function during the short time that the gas remains trapped by the bolt and the projectile. Once the projectile leaves the bore, the gas is bled off quickly.

The primary reason people try pigtails is to get reliable function is sbr's. A heavier buffer will almost always correct the problem; bolt bounce.

When the bolt returns home, the impact actually cause the carrier to back out of the lugs a bit. With the firing pin protrusion being only 1/64th of an inch during lockup, the bounce is just enough to keep subsequent rounds from firing.

Also, the heavier buffer slows the cycle down enough to not outrun the magazine springs. I've seen that happen on a few guns and the result is an empty chamber after cycling.
Link Posted: 1/27/2006 4:08:18 PM EDT
[Last Edit: 1/27/2006 7:48:22 PM EDT by Gunzilla]
It is important to keep in mind that on the 14.5" barrel, the dwell time is exactly the same as the 20" barreled rifle system, and on the 16" barrel carbine it is actually longer. This adds to the problem, as the maximum port pressure is much higer on the carbine length gas systems.

The critical pressure is the internal bolt pressure, which is a function of both port pressure and dwell...

One thing that is overlooked is the lock time of the rifle, on the carbine systems it is much shorter thatn that rifel and the weapon starts to unlock much sooner that it was designed to, or should anyway. Pigtails, U-shaped tubes and to a large extent expansion chamber designs all serve to "fool" the rifle into thinking that the gas port is located further out than it really is... the delay of pressure to the bolt caused by these systems does show some positive effect, but the down sides of them takes away from any measurable benefit.

As stated, on short barreled guns, the gas port must be opened considerably to get enough pressure to cycle the rifel, but bolt velocities are very high on these...
Link Posted: 1/27/2006 5:14:08 PM EDT
Tag- I've been considering putting together a mid length and there's some good info here.
Link Posted: 1/27/2006 11:30:06 PM EDT
Bump...
Link Posted: 1/29/2006 8:08:43 AM EDT
It seems as though the carbine may be less susseptable to underpowerd ammo since the PSI comming back into the reciever is a lot higher. Would this be correct?
Link Posted: 1/29/2006 8:51:35 AM EDT
Link Posted: 1/29/2006 9:04:44 AM EDT
[Last Edit: 1/29/2006 9:26:01 AM EDT by CCW]
Romad-556,

Thank you, thank you. I've been itching for this data since I bought my first AR15. I guessed it had to exist somewhere.

Thanks to Mark Westrom for publishing the data, and thanks to the AR15.com gang for providing a great place to find stuff like this.

Thanks also to A_Free_Man and all the others contributing to this thread. This is really answering alot of my questions about the mechanics and gas dynamics involved with the AR15 and variants.
Link Posted: 1/29/2006 11:40:38 AM EDT

Originally Posted By A_Free_Man:
Note in that Armalite Tech Note

"At a temperature of approximately 1100 degrees, however, the structure of this alloy undergoes a permanent transformation that substantially, and permanently, alters it."

Remember that one of the methods suggested by BATFE to permanently attach a flash hider or comp is to silver solder using a solder that melts at 1100 degrees. hr


Your point is valid. However, at that point on the barrel, any change in metallurgy might not be as significant since the pressure there is much lower than at or near the chamber of the barrel.

As a side note on temperature effects, I notice that an SEBR 14.5 barrel had been broken in rapidly by the new owner--to the extent that the stainless steel gas tube had permanently turned to a purple color as 3XX stainless normally does over 600F. The metal forestock bulkhead immediately behind the FSB had risen in temperature to the extent that the plastic contacting the metal piece had partially melted. I would have thought that the heat sink and free air cooling effect of the FSB would mitigate a hot spot at that point. But, such are the mysteries of heat transfer in the AR15 system.



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