Gas Port sizing on AR15 11.5" barrel

[Omega 5.56]

Does anyone know the recommended gas port sizing for a 11.5" barrel? I believe I'm having an under-gassing issue on my AR pistol and have read that the gas port should be between .086" and .094" for an 11.5", .750" diameter barrel. I measured my gas port and it is .070".

Any information anyone can provide would be great, Thanks!

 

[45calibre]

Have you tried a lighter buffer? Are you using a stock spring? Have you tried 5.56 ammo?

 

[Omega 5.56]

I'm running PMC XTAC 5.56, Heavy buffer, heavy extractor spring, Syrac Ordnance adjustable gas block, BCM gas tube. The bolt fails to lock back, even when the gas block is opened fully, does the same almost totally close. The common symptom I'm noticing is after a shot the the bolt will cycle and not toss the brass, so I'm thinking it isn't fully cycling, thus the under-gassing guess.

 

[45calibre]

have you tried a carbine buffer? so if you press the trigger a shot will go off but the case stays in the chamber?

 

[Omega 5.56]

Yep, already tried a Carbine buffer. And yes I loaded one round into a magazine, chambered it, and fired. After the shot the trigger will be reset, but the empty casing is still in the chamber. I know the bolt is definitely coming back at least some, I just don't think it's far enough because it should lock back on the empty mag.

 

[KDUBCR250]

I do believe my gas port is : .078-.080. Have you tryed you upper on a different lower and vise versa to make sure it's the upper ?

 

[digitalrebel80]

Try a different gas block if you have one.

 

[ViperJock]

Is your gas block adjustable? IMO adjustable gas blocks in sbrs FTW. Adjust it so that it barely runs with the SBR then when you put the suppressor on you'll get a touch more gas for reliability. My opinion. For what it's worth.

 

[ryknoll3]

He already said he had an adjustable gas block and the gun won't run even with the gas wide open.

 

[45calibre]

Omega, have you checked your bcg for a loose gas key?

 

[ryknoll3]

Also, is the gas block properly aligned with the port? Is there an equal-sized soot ring around the gas port?