AR-10 Gas System Issues

S

Stugotz

New member
I built/own 2 AR-10''s. My 308 rifle is no longer consistently ejecting the brass, and when it does, it is ejected out at the 2 o'clock position. (Over gassing I believe). Most of the time the shell case remains loose in the action with the bolt locking back and fall out the bottom with the Mag. when I hit the button. FWIW, I have a Syrac low profile adj gas block on a total of 4 rifles and 3 are working perfectly with the exception of this one.

I have been playing with the gas adjustment screw with little change other than when this is closed down, the action will not lock back. (Have been shooting this rifle as a single shot attempting to try and figure out the issue).

I did swap uppers with my 2 AR-10's (same lower) and the 6.5 CM upper functions perfectly with the lower locking back after the last shot and shell casings landing at the 4~5 o'clock position.

Any ideas on how to fix this problem?!? Thanks!
 
Sounds like too little gas is getting through. The bolt isn't going fully into counterbattery (the reason it won't lock back) and instead is starting its return into battery just as the brass clears the port, so the bolt starts forward and pushes on the base of the brass before it's clear (the reason it goes to 2:00) or just fails to go back far enough altogether (the reason it fails to eject).

Try cleaning the gas system. It may have carbon build-up. Slip 2000's Carbon Killer is probably the fastest acting on severe carbon. If you have time to wait, pour some Gunzilla through and wait a couple of week and see what comes out. Repeat either as needed.
 
I had been tinkering with the Adj. gas block and have ruled that out as the source of the problem. (BCG has always locked back). I had also checked the gas system and it is clear.

I have narrowed my focus to the ejector/extractor springs as the potential source of the issue.
 
From what was described ed in the OP, I don't believe this is a gas issue. I would check the ejector/spring. I've had an ejector start to seize up from dirt and grit before. After removal and cleaning, all was well. Your symptoms sound like an issue with the ejector.
 
To add to 5 Whiskey's comment: Are you loading on the zippy side?
When you clean your ejector look for little crescent shaped slivers of brass.
Those indicate either a slight burr around the hole in the bolt face,or,more likely,high pressure is causing a bit of brass extrusion into the ejector hole.
As the bolt opens,it is sheared off.
If you do discover these,you might back off a bit on your charge.

My old STG 58 (FAL) had an adjustable gas block ..original.
The procedure was to load one round in the mag.Close the valve down till the bolt would not stay open on the last round.
Then open it one click at a time till the bolt would stay open last round.
Add one more click open for reliability.Done.Gas is set.
From there,ejector function is another subject.
One other sneaky issue could come up.
On an AR-10 type rifle,if it has a carbine stock,There are two lengths of buffer .
I don't know the exact lengths,but think 2 7/8 vs 3 1/4 .Long buffer in a tube that needs a short could be a problem.

Even sneakier! There was the A-1 fixed stock.Oldschool.Then the A-2 stock came along.Its 5/8 in longer.They added a 5/8 spacer behind the buffer tube,and used a 5/8 longer screw.
If you put an A-1 stock on and use an A-2 length butt screw,your buffer smacks the too long screw.

One other thing to check is port alignment.It is not necessarily true that the gas block belongs against the shoulder on the barrel.Something about military type versus free float tubes.I forget the details.
 
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No reloads shot with this rifle yet. Factory Federal 150 gr soft point, Rem 150 Core-Lokt, Federal 168 gr Sierra HPBT Gold Medal Match which has been the majority of ammo fired in this rifle. (When I start reloading for this rifle I will be basically duplicating this round).

"When you clean your ejector look for little crescent shaped slivers of brass".
Shot it yesterday afternoon (30 rounds) shooting only the factory 168 gr FGMM.

I also was up at the lake home last weekend shooting this rifle. after I finished, I cleaned the action and cleaned and lightly lubed the bolt BCG. Basically had the same results at yesterdays range session. I will be be duplicating the cleaning this afternoon on the BCG and will look closely for any slivers of brass. (Thx for the suggestion).

"The procedure was to load one round in the mag.Close the valve down till the bolt would not stay open on the last round. Then open it one click at a time till the bolt would stay open last round.
Add one more click open for reliability.Done.Gas is set".

This is the process that I have followed with all four of my AR systems. ;)

"On an AR-10 type rifle,if it has a carbine stock,There are two lengths of buffer . I don't know the exact lengths,but think 2 7/8 vs 3 1/4 .Long buffer in a tube that needs a short could be a problem".

I do have 2 different AR-10 lowers each set up with a MEGA lower, identical small parts kit, and Geissele Nat Match FCG. The only difference beyond that is I am using a Magpul UBR on one and a Magpul PRS Butt stock on the other. (Each of those complete lowers is set up with the corresponding JP Silent Captured Spring system with tungsten weights used on the PRS). I have interchanged the uppers with the lowers (making adjustments to the adj. gas blocks on each) with the same results on the 308.

"One other thing to check is port alignment.It is not necessarily true that the gas block belongs against the shoulder on the barrel.Something about military type versus free float tubes.I forget the details".

I broke down the gas system removing the block/tube etc and re-assembled last Sunday. (Shot it yesterday with same results?.

Replacement parts for the BCG should be received today and I break the BCG down and re-assemble with the new parts. Thanks again for the suggestions! :cool:
 
For the OP: Why do you think it's the gas system and why do you think the adjustment screw is the only part of the gas system that could be wrong?

Ejection angle, assuming the bolt cycles, is not usually a gas system issue. Ejector and extractor are the places I'd look. Especially the extractor.

Jeff

Sent from my XT1254 using Tapatalk
 
To be honest, I completely eliminated the gas system after I tore it down last weekend. It was just one of the things I was looking into. I had also contacted the OEM manufacturer of the BCG and discussed my concerns about the extractor, springs and ejector. I have received a new extractor, ejector, and ejector spring and will be replacing today.

Unclenick was the 1st one to respond to this thread and he proceeded down the path of a 'fouled' gas system. I responded to his post stating that I ruled out the adjustable gas block as a problem... (This was back on the 25th).

In that same post I made, I stated the following:

"I have narrowed my focus to the ejector/extractor springs as the potential source of the issue".
 
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