AR 308 vs. Armalite AR10 rifle length gas tube specs

[QuangTri]

Anyone having issues with cycling with a .308 rifle length gas system, you may be using a too short AR tube.

Just spent too much time figuring that out. Went from a spastic cycling gun to a very consistent one switching to an Armalite rifle length gas tube. Only a difference of .250 -.375", but a major improvement.The shorter AR tubes will cycle, but not be reliable.

There is no specification for gas port location on AR .308 barrels as most know. Hence the gas tube issues.

 

[BigNate]

Thanks [mention]QuangTri[/mention]

TNA makes AR-10 tubes - they are listed here: https://truenortharms.com/ar15_default_store_view/gas-tubes-for-ar-15-ar-10-stainless-1.html

The summary is as follows:
Gas tube size Length in inches
AR-15 Midlength 11.75"
AR-10 Midlength 12.50"
AR-15 Rifle 15.125"
AR-10 Rifle 15.375"

FWIW - on my pistol length AR-10 build I believe that I used what I understood to be a standard pistol length tube. It functions nicely. Now I want to tear it apart and measure it... LOL

 

[QuangTri]

"pistol length AR-10 "

Holy muzzle blast Batman!


The Armalite/AR10 rifle length I had found was 15.5. 15.375 seems like it would work the same.
15.5 is just slightly past the midpoint of the cam cutout so the 15.375 might be perfect.

My big issue was the carbon ring that had set up in the gas key with the too short tube. It was preventing the bolt going into battery.
Have added scrubbing the key to clean up chores.

 

[BigNate]

"pistol length AR-10 "

Holy muzzle blast Batman!

8.6 BLK... pretty much exclusively subsonic stuff... it's really quite mellow... ;-)

 

[xerts1191]

https://www.youtube.com/watch?v=7yW_ITuR7nU
Seems appropriate for .308 discussions

 

[Rock Hardson]

I bought one of these in March. Now Adams Arms has gone out of business, which sucks because I wanted to pick up a 16" P2 .308
https://youtu.be/ig037_Gae7g?si=Ps-iyjRMC-luUSeI

 

[Suck My Glock]

Yep, all of this is one of the "pros" for going with a piston system.

Not all powders have an appropriate pressure curve for cycling direct impingement systems. We've seen this in the 5.56 rifles most famously with the old Tula steel case ammo. Russia did their load development with 5.56 AK rifles, not DI Stoner design gas systems. When the ammo worked reliably in what they tested it in, that was good enough for them and they called it a day. Many fellas had cycling issues in their direct impingement AR15s with the Tula ammo, but found it worked perfectly fine in piston-driven ARs.

.308 utilizes an even broader spectrum of powder burn rates. Not all of it is designed to produce an appropriate pressure curve to operate AR rifles.

There is nothing inherently "wrong" with DI guns. But they do require attention to ammo selection to ensure reliable operation. Piston-driven guns are simply less sensitive to this.