New One Today

[Ronin22]

Before "Why doesn't my AR10 Work?" I self rescued.
Quick version. AR10 708 built 6yrs ago and 600ish rounds. Out today to run 6 different recipes through it, and all 6 gas settings established, which was 6-7 clicks out from closed on an Aero Adj GB. (Restrictive style)
I move onto POI tests. The gun went into single shot.
Turns out the daggum gas tube roll pin sheared off.
Ya'll ever see this happen, or am I the only lucky one? (Or, know why it happened?)

 

[msgriff]

A couple things that you probably would have noticed before now would be improper gas tube alignment with the gas key or too long/improper length gas tube, which could drive the gas tube forward breaking the spring pin. Other than that, I would look at something like carbon build-up in the gas key, again driving the gas tube forward. There are three steps, or inside diameters, in the horizontal part of the gas key. The first step is the part that interfaces with the gas tube, and is spec'd out as an inside diameter of 0.1806" (+0.0008") for the first 0.820".

 

[Ronin22]

Yup, I had thought the tube had to be getting forward force applied and after time the roll pin gave out. The barrel, tube, and pin are from Wilson Combat. The tube is 3/4 way into the cam pin cut out of the receiver like "they say" it should be. I'll get into the gas key and try to scrape out any possible carbon build up. Trying to think of a way to identify if the key is too shallow, like small piece of tape on end of gas tube to see if there is an indent from touching the back of the key.

Maybe the pin was initially on the short side. The Aero GB has more width than a STD low pro block. The new pin flushes out with each side of the block. I made damn sure of that this time!

 

[msgriff]

Yup, I had thought the tube had to be getting forward force applied and after time the roll pin gave out. The barrel, tube, and pin are from Wilson Combat. The tube is 3/4 way into the cam pin cut out of the receiver like "they say" it should be. I'll get into the gas key and try to scrape out any possible carbon build up. Trying to think of a way to identify if the key is too shallow, like small piece of tape on end of gas tube to see if there is an indent from touching the back of the key.

Maybe the pin was initially on the short side. The Aero GB has more width than a STD low pro block. The new pin flushes out with each side of the block. I made damn sure of that this time!

If your barrel has rifle length gas, there is a chance that the gas tube is too long and is bottoming out in the gas key. Generally, there are two different rifle length gas systems for large frames. DPMS pattern uses the same gas tube as an AR-15 and is ~15.125" long, and ArmaLite AR-10 pattern uses a ~15.5" long gas tube. With both pattern barrels using the correct corresponding gas tube, the gas tube should extend around 1" into the upper, or halfway into the cam pin cut-out. Wilson Combat is known for placing their gas ports different than everyone else (Faxon does it too), halfway between the two common patterns, and the longer ArmaLite pattern gas tube can bottom out. One way to check is before you install the spring pin. Use a temporary smaller diameter pin or wire, then with the BCG in battery, remove the wire and see how far you can slide the gas tube to the rear, looking at the alignment between the holes in the gas block and the gas tube. The tape method might work, but it wouldn't take much interference with repeated hammering before there could be an issue. Also, spring pins, particularly for the gas tube/block, are considered one time use only.

 

[Ronin22]

Solid advice. I distinctly recall the WC gas port thing and got their tube to help compatibility. But the Aero BCG is set up for DPMS length and most likely the gas key depth. Makes complete sense now. So, with bore scope to make sure I have an unobstructed gas port, I managed a .025 gap between the journal shoulder and GB. I'll be hawking the tube to make sure It's not hitting.

 

[LRRPF52]

Check the flange on your gas tube that interfaces with the carrier key.

If there is wear on the flange, it means it’s clipping the key.

I’ve seen plenty of AR-10s and AR-15s with RLGS now that are more prone to carrier key clipping.

The constant impact of that would be my first suspect in shearing the gas tube roll pin.

 

[Ronin22]

So, I had a AR10 15.5" in the gun when the problem occurred. There is not sign of damage/peening of the gas tube opening gas key side. That 15.5 tube went 3/4 way into the cam recess area. I put a 15.125 in right now and the tube is 1/4 way into that recess area. As @msgriff mentioned. Wilson Combat gas ports are between the two. So, WTF is a guy to do? I am gonna function test the 15.125 tube in a minute. If it runs good, I guess too short better than too long, because we all see what happened with too long.

 

[LRRPF52]

So, I had a AR10 15.5" in the gun when the problem occurred. There is not sign of damage/peening of the gas tube opening gas key side. That 15.5 tube went 3/4 way into the cam recess area. I put a 15.125 in right now and the tube is 1/4 way into that recess area. As @msgriff mentioned. Wilson Combat gas ports are between the two. So, WTF is a guy to do? I am gonna function test the 15.125 tube in a minute. If it runs good, I guess too short better than too long, because we all see what happened with too long.

You need a tube from Wilson or a custom-made tube.

 

[Ronin22]

You need a tube from Wilson or a custom-made tube.

This 15.5 tube came from Wilson combat. WC website lists length of their AR10 gas tubes 15.5.
So, where does one get a custom gas tube?

 

[msgriff]

This 15.5 tube came from Wilson combat. WC website lists length of their AR10 gas tubes 15.5.
So, where does one get a custom gas tube?

White Oak Armament makes custom length gas tubes. From what I remember, the default gas tube on these barrels used to be the AR-15 length. Then if you experienced short stroking that was truly caused by being under-gassed, you would either try the ArmaLite length gas tube (making sure that is wasn't too long), get a custom length gas tube, or open up the gas port one size. You could also readjust the AGB, if applicable, or try a lighter buffer weight. Typically, even with the shorter AR-15 gas tube, the bolt has completely unlocked and has started the extraction process before the gas tube disengages from the gas key. Also, the offset bend of the gas tube required for a large frame is different than an AR-15, and depending on the gas block/AGB, it will need to be adjusted for proper alignment with the gas key.

 

[LRRPF52]

This 15.5 tube came from Wilson combat. WC website lists length of their AR10 gas tubes 15.5.
So, where does one get a custom gas tube?

You can get an Extended Length Gas Tube and have a machinist cut it down, re-plug the front, cut a new gas hole in position, and a new roll pin hole. They will need to take measurements necessary to land the gas tube into that space relative to the cam pin recess in the upper so that it’s midway to that space.

I’ve had it done for me before. My 17.6” Grendel is actually MLGS -0.625” I think. I had it short-chambered and an extension re-installed. It started out at a little over 18” from Lilja.

The reamer-supplier purposely sent Lilja a non-spec reamer, when we really asked for a SAAMI reamer, so I sent it to Bill A. and he squared it away for me. That’s the whole reason why my 17.6” Lilja is that weird length, and not 18.x”.

 

[Ronin22]

15.125 tube was a failure
15.5 tube reinstalled, but with .025 off barrel shoulder, which aligned perfectly with gas port as seen in borescope. 3 of 4 powders ran fine with 3:00 ejection. (IMR3031, AA2520, N540)

N135 would not. I’ll drop from 5.3oz heavy buffer to reg weight carbine buffer for this load. It’s the most accurate load I made too.

 

[Ronin22]

Dropping to 3,3/4 oz buffer did the trick, for now. Since I don’t see damage on the 15.5 tube I’m gonna run it. Keep an eye on that roll pin.

But something changed within the system. Ammo that I thought was loaded too hot, now shooting great and over pressure signs gone.

Gas setting migrated to more open settings. 14 clicks out now, 7-8 clicks out previously.