2nd/3rd round FTE/FTC

[BennyO]

Hello -
Encountered a strange functional problem today hoping someone can help with. Custom built AR-10 chambered in .308, 18" barrel, SA adjustable gas block. When shooting the rifle unsuppressed it runs great, tried different .308 loads, no problems (except variations in accuracy due to bullet weight). Installing the suppressor which is a Dead Air Sandman-S using the Keymo mount system, the 2nd round fails to extract fully from the ejection port and the 3rd round gets pushed up underneath the 2nd when trying to chamber but obviously fails and totally bends the casing on the 3rd bullet. Magpul 10-round magazine is what I am using, didn't have any other magazines with me to try.

Tried different ammo brands (PMC, Federal, Freedom), all did it on the 2/3rd round when suppressed. Tried turning up the gas block but that didn't seem to help. We switched back several times between suppressed and unsuppressed and could reproduce the problem. Thinking it is the magazine, gas system or something else I haven't discovered yet when shooting suppressed. Another theory was some type of gas stall in the barrel causing the failure but I don't even know if that's a thing. I don't think it's directly the suppressor as the suppressor runs fine in other 5.56 rifles, pistols and .300 blackout.

I wish I had taken pictures but I didn't. Rifle was cleaned thoroughly before this range session.

If anyone has ever experienced this or have any insight I'd appreciate your feedback.

 

[mark5pt56]

Check extractor tension as it sounds as if you are slightly over gassed and the probable weak extractor spring is being over worked. You have more gas flow with the suppressor so this may cause bolt unlock and movement prior to chamber pressure being lowered. You may also need to tweak the block to even things out.

Basically, the extractor jumped the rim and the next round is trying to get it there--also known as a SEAL party-gotta make room for the next one in!

 

[rpoL98]

when you go from un-suppressed to suppressed, you need to turn the gas down, not up.

 

[BennyO]

Thank you both for your feedback, I'll turn the gas down and try again.

 

[msgriff]

What recoil/action system are you using and what is your buffer weight?

 

[BennyO]

Standard buffer tube with collapse stock, spring and buffer are just your normal run of the mill parts, nothing special.

 

[msgriff]

Standard buffer tube with collapse stock, spring and buffer are just your normal run of the mill parts, nothing special.

Not much in the way of "run of the mill" action/recoil systems when you're dealing with AR-10/SR-25/AR308 firearms. Posting the actual parts description/specs would help in identifying any potential issues. Are you familiar with adjusting the Superlative Arms AGB? If you are beyond 4-1/2 rotations CCW from closed, you're in "bypass" mode. Further turns CCW will result in less gas being delivered to the BCG.