Lessons Learned from an Adjustable Gas Block on an AR308: Jamming on Extraction

[rchman]

This is more of an FYI, since I see this question posed a lot about 308ARs, but my previous few builds jammed when they were new (failed to eject) and required a great deal of force to unjam. I could never figure out why, although I knew it happened almost always with 147gr ammo whereas FGMM 168 would run reliably.

So, the answer? It seems if the rifle is undergassed, it can get stuck in the extraction phase and jam, possibly requiring you to break out the screwdriver to pry the bolt back.

I noticed this when I got my PRI gas block and started with no gas and worked my way up. About mid way through, the bolt would get stuck real good. Once I opened the gas block up further, it was back to reliably extracting and feeding.

Hope this helps some people out there.

 

[Oryx]

This is more of an FYI, since I see this question posed a lot about 308ARs, but my previous few builds jammed when they were new (failed to eject) and required a great deal of force to unjam. I could never figure out why, although I knew it happened almost always with 147gr ammo whereas FGMM 168 would run reliably.

So, the answer? It seems if the rifle is undergassed, it can get stuck in the extraction phase and jam, possibly requiring you to break out the screwdriver to pry the bolt back.

I noticed this when I got my PRI gas block and started with no gas and worked my way up. About mid way through, the bolt would get stuck real good. Once I opened the gas block up further, it was back to reliably extracting and feeding.

Hope this helps some people out there.

So are you saying that it was under gassed because the adjustable block you installed was not adjusted correctly or with the stock gas block? Installing an adjustable gas block does take some initial tuning to get correct (or when you change ammo with a significant change in pressure). Maybe I'm reading wrong?

 

[ORD]

Not so sure I necessarily agree with your conclusions re: undergassing or the broad generalization that this is somehow a "common" problem among .308/7.62 gassers or if that was even the root cause of the issues you experienced. In fact, most of the time, the exact opposite is true inasmuch as factory semis tend to be OVERgassed right out of the gate and even WORSE once you start shooting them suppressed.

An adjustable gas block will certainly work to resolve BOTH undergassing AND overgassing far better/more effectively than what I see a lot of people doing with buffer weight swapping, XP/lower power buffer springs, carrier weights (or lightened carriers), etc.

 

[rchman]

[MENTION=96262]Oryx[/MENTION]
The instructions I followed for the gas block said to turn the block completely off, then gradually open it up until it fully cycles. Once I got to the point where it half cycled, lots of problems began.
[MENTION=6412]ORD[/MENTION]
While I agree with that statement, my experience with 3 recent MA-TEN's have been different. The break-in period was terrible if anything other than FGMM was used (or probably other 168gr rounds). My last MA-TEN with a Noveske barrel and armalite bcg had this problem (not fully cycling & bolt getting locked) until 300 rounds as well as two with Rainier barrels. Ball ammo is still a no go for these rifles (even my current Krieger barreled MA-TEN).

Now, I've had two AR10's (Armalite) that ate everything, and a POF-308 that never complained.

I should have specified that my experiences have been related to newly built rifles with less than a few hundred rounds down the tube. There's a handful of people with the same symptoms in the MA-TEN thread on ARFCOM. It could be that the tolerances are initially tight, making them slightly undergassed. Once they break in, they're properly gassed for full powered rounds.

 

[Oryx]

I would go about it the opposite way and start with it fully opened up and start working your way down until it won't cycle consistently, then open it back up. Partial cycling will wreck the brass as it gets pulled out of the chamber, doesn't eject completely, and then it tries to close the bolt on it - the necks will get destroyed and all sorts of jamming can occur.
It's really a learning process, but a quick one. New loads may require adjustments if your on the edge of the proper working pressures.

 

[BgBmBoo]

Hmmm.....pretty much every large frame AR...308, 6.5CM, .243..... That I ever had was waaayyyyyy over gassed. Maybe yours is a anomaly.

 

[coryhenry]

I just went through this process on my new .308. I followed the instructions and put one round in the magazine with the gas block closed, then opened it one quarter turn each shot until the bolt locked back. I never had any extraction problems.

 

[bunsen27]

[MENTION=46057]rchman[/MENTION] It sounds like your gas block instructions are missing a key piece of info/step:

"...turn the gas adjustment screw in all the way to close it off. Then, back it out approximately two full turns, and load one round in the magazine and fire. If the bolt holds open, the gas block is set. If the bolt does not stay open, it is short-stroking, and the valve should be opened about another half turn. Continue backing the gas adjustment screw out until the bolt holds open consistently on last round. Test this again with one round in the magazine."

Via JP Rifles whose adjustable gas blocks are on all my AR's including my LR-308 which has worked flawlessly for thousands of rounds. Once set up/tuned it's remarkable how "soft shooting" most AR's will feel, especially coupled with a good muzzle break.

http://www.jprifles.com/document_pdfs/JPGS1_516.pdf

 

[rchman]

Ya, I was definitely missing that step.

Either way though, through missing that step, I noticed I was able to create a problem that I had with my last MA-TEN during the break in phase. That was my point of posting this, to help anyone else identify the issue in case they were experiencing the same symptoms. When I posted about the problem last year everyone wanted to blame the extractor or bolt. My problem went away on its own with that rifle. This rifle never had that problem.

 

[KYbulldog]

Armalite uses .099 gas port for SASS(rifle length), .0845 for non SASS std 20" barrel. I did order a 18.5" rifle gas barrel with the .099 port size but I never checked it because it ate everything I put in it, and all with a std. Larue gas block without beating me or the gun up. I recently installed an SLR adjustable gas block and I have to open 10 "clicks" (of the 15 total) for proper function. That said it doesn't tell much unless I know what the actual port size is. Now I'm curious….