Ar15 Gas tube Roll pin

[pipepro]

I was shooting my AR pistol today and it started acting up after about 60 rounds. Failure to eject, wouldnt cycle far enough to even try ejection.

I had just changed the gas tube to a smaller gas port to help the original overgassed condition. I mistakenly started thinking it was way under gassed and pulled the H2 buffer and Blue spring out and swapped in a standard Carbine buffer and stock spring. No joy. Still a failure to eject on every round.

I went home thinking I had blown rings in my bolt and I didnt have a spare with me. I noticed lots of gas blowback on the barrel by the gas block so I pulled the handguard to check it out. Low and behold the brand new roll pin that holds the gas tube into the gas block was sheared off! Not fell out it was sheared off and a piece of it was still in the gas tube!

The Tube wasnt being held in place and the small gas port wasnt lined up so no gas going to drive the bolt, presto.

Now I found the issue but what is the cause?

I thought maybe the gas tube was too long and the bolt gas key was hitting the tube and pushing it forward. I compared it to the one I removed and they are Exactly the same length. I also put the BCG into battery and slid the tube in as far as it would go. There was still 3/32" of wiggle room so it doesnt appear there was any way the BCG could hit it forward. I checked the gas key end of the tube and I dont see any damages there either. Its an Aero precision enhanced upper so it has a long section that the gas tube travels through so it was aligned and I cant see any way to mis-align it.

Is it just a fluke? A bad roll pin? Anyone have this kind of thing happen, and what did you figure out?

I cleaned everything up and put a new pin in it tonight. Ill try and shoot it tomorrow. I hope it was just a one time deal.



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[slingshot77]

When you originally installed the new gas tube did you verify that it smoothly entered the bolt carrier gas key as the bolt carrier went forward?

It seems that if the tube isn't bottoming out in the gas key when in battery, and the gas tube was properly aligned with the gas key so it entered smoothly and properly, then about all that remains was a defective roll pin or a very loose tube fit in the gas block.

Is the gas tube front end where it enters the gas block a fairly tight fit? If that interface has a lot of vertical free play then possibly the gas pressure from the smaller hole pushed the front of the gas tube up enough to shear the pin.

Can you tell from the damage to the pin what way it was sheared? If it was sheared by going forward then probably a gas key contact issue. If it was sheared by going rearward then possibly a front pressure issue. If it was sheared in the vertical direction then possibly a loose fit with excess vertical gas pressure due to that smaller gas hole.

Personally, I have never used a smaller gas hole gas tube as I usually just use an adjustable gas block.

[pipepro]

All good points.

I did verify the BCG would smoothly engage the gas tube.

Its not a super tight fit but it fits with no appreciable wiggle.

Looking at the pics, I would almost say that it was sheared going rearward. As you said, a front pressure problem. I have no idea how gas pressure could migrate in front of the tube and act on the flat end but I cant rule it out. I imagine the tube could have been stuck in the gas key and when the bolt went rearward so did the tube. I still cant grasp exactly how that happened mid outing but I cant rule that out either.

I cleaned everything, and installed a new roll pin. I took it out this AM and everything works perfectly. Locks back on an empty mag, Ejection is right at 3:30 and the recoil pulse is not as harsh. I put 150 rounds through it and other than it being a bit dirty there were no issues. An already overgassed 10.5" gun and then I added a Turbo K on it and it was severely gassy.

Im going to go with the idea that it was a bad pin. BUT, Im going to keep an eye on it. Ill run another 150-300 through it this weekend and if no issues Ill have to chalk it up to a fluke.

[slingshot77]

All good points.

I did verify the BCG would smoothly engage the gas tube.

Its not a super tight fit but it fits with no appreciable wiggle.

Looking at the pics, I would almost say that it was sheared going rearward. As you said, a front pressure problem. I have no idea how gas pressure could migrate in front of the tube and act on the flat end but I cant rule it out. I imagine the tube could have been stuck in the gas key and when the bolt went rearward so did the tube. I still cant grasp exactly how that happened mid outing but I cant rule that out either.

I cleaned everything, and installed a new roll pin. I took it out this AM and everything works perfectly. Locks back on an empty mag, Ejection is right at 3:30 and the recoil pulse is not as harsh. I put 150 rounds through it and other than it being a bit dirty there were no issues. An already overgassed 10.5" gun and then I added a Turbo K on it and it was severely gassy.

Im going to go with the idea that it was a bad pin. BUT, Im going to keep an eye on it. Ill run another 150-300 through it this weekend and if no issues Ill have to chalk it up to a fluke.

I suppose it is slightly possible that the extra gas pressure on the bottom of the gas tube OD, due to that smaller hole, might have allowed the tube to move up within it's play leaving just enough gap on the bottom to allow the gas pressure to get under the tube and migrate around to the front (in theory anyhow)

The good news might be, if you can get the new pin to live long enough then the tube to the gas block interface will probably carbon up enough to seal the tube to the gas block.

You might get another new pin then remove your current pin at about 500 rounds to see if there are any witness marks indicating that it is seeing some shearing forces.

If you still see any signs of shearing then maybe drill a bleed hole in the front of your gas block. I have a number of gas blocks that don't even have a closed off front (gas tube hole goes all the way through to the front)

[pipepro]

Ahh, thats a great idea. Im looking into getting a few of the pins to keep on hand. I am also going to drill a small hole as you suggested in the front of the gas block. That will for sure remove any chance of gas pressure building up there and it will show the blowby if there is any. Thanks for the ideas!