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Tuning assistance needed - FTF, FTE and more


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Tagging @98Z5V as I think this might be a gas port issue.

Background: I just assembled 2 new identical 308AR rifles.  They have all the same parts as the previous one I built, which fired perfectly after I changed to the Sprinco spring/KAK buffer combination.  But, I never measured the gas port on the previous build, so I wonder if it may have been larger.  When measuring these two barrels, they definitely seem to be at the low end of the acceptable range, but want to ask here before doing any drilling(and get a size recommendation).

Issue: Both of these rifles are exhibiting the same issues.  The first 5 or so shots worked fine.  After that, I have had failure to feed after almost every firing, failure to eject on a few, and 2 stovepipes.  Ejection angle is consistently at 4 o'clock.  I decided to stop after about 20 rounds, as this seems to be more than just a "needs breaking in" scenario.


Waterboarding Info:

1.) Do you have Armalite AR10 Recoil System:

2.) buffer weight
5.1oz (KAK LR-308)

3.) buffer length

4.) Extension Buffer Tube internal Depth

5.) What Spring, Length, Coils, Brand, Color
Brand: Sprinco Orange
Length: 11-5/16"
Coils: 29

6.) Gas System Length- Rifle, Midlength, Carbine, Pistol, Other?
Rifle length


7.) Gas tube Length
14-5/16" plus portion inside gas block (approximately 15-3/16")

8.) Photo looking directly into the magwell


9.) Photo of the Bolt Face


10.) BCG Clearance at the ears and bolt stop



11.)  Gas Journal Diam and gas Port Diam
Gas Journal: .750"
Gas Port:
 Go: .0860"
 No-Go: .0890"

12.) Photo and Description of the Malfunction or Issue
Issue described above

13.) More Photos

14.) Additional Information
Upper: Aero M5 18" .308 CMV Complete Upper Receiver w/ ATLAS S-ONE Handguard
Barrel: Ballistic Advantage
Lower: 80% Arms
BCG: Aero 308/7.62 with 9310 bolt, black nitride
Magazine: Magpul PMAG 20
 175gr Bullets 1st .308 Win hollow point
 178gr Hornady .308 Win Hornady ELD-X
Lubrication: It is sopping wet.

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Nice job on the waterboarding. 98 is the gas port whisperer, but my comment addresses the last 2 photos. Looks like you have a very large amount of distance from the bolt stop to the bolt, but I assume by the last photo the BCG is not contacting the ears. My builds usually only have 1/8" from the bolt catch to the bolt face. I don't have a conclusion about cause. Second thing, factory ammo? I did have issues using my bolt/ lever gun loads in my first AR 308. Too much pressure at the port. Even with all other things correct, still had issues until I went to AR Comp powder. 

I have had good success with a KVP 2.5" buffer and orange spring, but I think it is 5.6 oz and also running AGB.

Edited by Albroswift
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3 hours ago, RacerX said:



11.)  Gas Journal Diam and gas Port Diam
Gas Journal: .750"
Gas Port:
 Go: .0860"
 No-Go: .0890"




14.) Additional Information
Upper: Aero M5 18" .308 CMV Complete Upper Receiver w/ ATLAS S-ONE Handguard
Barrel: Ballistic Advantage


It's right here - you're right, it's gas port diameter.  18" rifle gas with a 0.750" journal size needs 0.093"~0.096" to run right.  :thumbup:

What you have now, at 0.086" is just barely on the high side to make an 18" midlength gas barrel run, with a 0.0750" journal.  There's alot less dwell time over the midlength, using the rifle length, so you need more gas port diameter to make up for that.

Edited by 98Z5V
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