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AR15 Failed Gas/Carrier Key

1155 Views 36 Replies 16 Participants Last post by CobraBG, 8 d ago

Soganofla

Discussion Starter · 15 d ago

Had a strange occurrence happen the other day involving a newly-built, never fired AR15 (complete PSA lower with complete Bear Creek upper).
The rifle was disassembled, cerakoted and reassembled - including the BCG. Passed all unloaded functions tests and was shipped back to the owner.
First time at the range it fired but did not eject properly. Manually ejected then test fired again with the same result. Broke the gun apart and found the problem to be a split gas key.
I had the owner ship the BCG back to me for a complete disassembly and found nothing wrong other than the split key. The owner also checked the gas tube for any blockages/issues and found none.
I want to figure out what could have caused this - if anything other than metallurgical failure of the gas key.
Thoughts?

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Nilla

· #2 · 15 d ago (Edited)

If it was 1st time shooting like you said it was, looks like a metallurgical failure good thing it’s a replaceable part and didn’t happen when it was a critical time. Bear Creek complete upper sounds like there’s your problem. I don’t have any experience with there product but I’ve read on forums they have QC issues.

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Mpddoud162

· #3 · 15 d ago

thats definitely some stressed metal. Is there a possibility that the key was blocked/plugged to keep the cerakote out of it and that plug left in it causing a blockage? If it didn’t eject the fired round you can assume the crack was already in place and opened up upon the first shot, or the first shot caused the split because of restricted air flow, but you would still be getting some gas passing through the key as it should. The key needs to come off anyways so you may find the reason then.

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Soganofla

Discussion Starter · #4 · 15 d ago

Mpddoud162 said:
thats definitely some stressed metal. Is there a possibility that the key was blocked/plugged to keep the cerakote out of it and that plug left in it causing a blockage? If it didn’t eject the fired round you can assume the crack was already in place and opened up upon the first shot, or the first shot caused the split because of restricted air flow, but you would still be getting some gas passing through the key as it should. The key needs to come off anyways so you may find the reason then.

Gas key inlet was blocked prior to blasting and coating.
After blasting plug was removed, high pressure air was blown over and through the entire part, then the plug was reinserted into the tube inlet and it was coated. Plug was removed after coating/cooking and BCG was reassembled.
Dry fire functions test was performed after rifle reassembly. Gas tube freely moved to the rear of the gas key allowing for the BCG lugs to lock up.
FWIW, I was speaking with a gun builder when I received news and pictures of the problem and his first thoughts were "bad part" or "metal failure". I'm not looking to pass the buck - just a viable solution.
I remove the key and found no issues other than the split.

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sigamore

· #5 · 15 d ago

My thoughts are 1) Bear Creek Arsenal is the bottom of the barrel, and 2) the failure occurred because of the added dimensional thickness of the cerakote, which probably caused added friction and the first part to feel and be impacted by the added resistance (and resulting pressure increase) was the gas key. I purchased a Brownell's nickel boron coated bolt carrier group (308AR BOLT CARRIER GROUP NIB - Brownells #078-000-392) a few years back that could easily be cycled by hand but would not pick up a second round and sometimes had a failure to eject. From my gun-build notes for that particular 20" Aero Precision M5 308 build, "Sanded coating on BCG, seemed excessive. Ran 25 flawless rounds on Sept 28, 2020. Cycle, feed, extract, bolt hold back all good!!"

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Soganofla

Discussion Starter · #6 · 15 d ago

sigamore said:
My thoughts are 1) Bear Creek Arsenal is the bottom of the barrel, and 2) the failure occurred because of the added dimensional thickness of the cerakote, which probably caused added friction and the first part to feel and be impacted by the added resistance (and resulting pressure increase) was the gas key. I purchased a Brownell's nickel boron coated bolt carrier group (308AR BOLT CARRIER GROUP NIB - Brownells #078-000-392) a few years back that could easily be cycled by hand but would not pick up a second round and sometimes had a failure to eject. From my gun-build notes for that particular 20" Aero Precision M5 308 build, "Sanded coating on BCG, seemed excessive. Ran 25 flawless rounds on Sept 28, 2020. Cycle, feed, extract, bolt hold back all good!!"

There were a few other "issues" with the Bear Creek upper build that were unrelated to this but I was not impressed with some of the assembly.
The added dimensional thickness of the cerakote (.5mil) should not be an issue as the gas tube goes inside the gas key which received no coating and the only point of contact would be the bottom of the bolt where the hammer rides under spring tension. Not disputing your thought process - just want to make clear the points of contact.

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dghal

· #7 · 15 d ago

My guess,

It was just a dud from the day it was made, It can happen,

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Soganofla

Discussion Starter · #8 · 15 d ago

dghal said:
My guess,

It was just a dud from the day it was made, It can happen,

That is what I want to believe but would like to rule everything else out - just in case.

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Devereaux

· #9 · 15 d ago

Not sure there is anything else to add. IF there had been some obstruction to the tube part, the rifle would have failed the dry function test. If, as @sigamore notes, there was a friction issue, as it seems he had, you would not have seen a split tube - just poor function when firing. You appear to have followed all the proper steps in doing what you did. Can’t account for bad metallurgy. That can happen to anyone - even quality parts, although the odds decrease (but not to zero).

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sigamore

· #10 · 15 d ago (Edited)

Soganofla said:
The added dimensional thickness of the cerakote (.5mil) should not be an issue as the gas tube goes inside the gas key which received no coating and the only point of contact would be the bottom of the bolt where the hammer rides under spring tension. Not disputing your thought process - just want to make clear the points of contact.

Absolutely understand but it looks like the complete "carrier" was coated, and that is what I would suspect caused the additional friction/pressure increase, as with the NiB coating on the Brownell's carrier that I referenced. The carrier key is bolted to the carrier and, if the carrier experiences any additional resistance in movement, a pressure change will occur, and the key would have been the first recipient of that pressure change as the interface between the gas exiting the gas tube and entering the carrier key. A very slight/light wet sanding of the carrier coating is what returned my BCG to full functionality, as I stated above.

You were looking for possibilities, right, or are you just looking for some other fault other than the applied cerakote and/or BCA quality?

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P938365

· #11 · 15 d ago

Soganofla said:
That is what I want to believe but would like to rule everything else out - just in case.

Everyone who pointed to metallurgical failure is on point. I can say from experience that a blockage will not result in a split gas key tube. Years ago, my motor sergeant was issued a theater-provided M-4 that would not cycle, other than manually. When my armorer sent it to the third-shop for diagnosis, the mindless fools there only gauged the weapon, so they failed to find the problem.

When it came back with the problem unsolved, my armorer brought the weapon to me, and one of the first things I did was run a pipe cleaner into the gas key, at which point I discovered a plug of q-tip wedged in there. It was in there hard enough that I had to use a sheet metal screw to bore into the mass and pull at it with a Gerber tool, while one of my NCOs pulled the bolt carrier from the other side. We basically had to tear chunks from the mass incrementally before we got it to the point that the rest of it could finally be fully extracted.

Despite that blockage, the gas key's tube did not split. It only resulted in a failure to cycle at all.

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Soganofla

Discussion Starter · #12 · 15 d ago

sigamore said:
Absolutely understand but it looks like the complete "carrier" was coated, and that is what I would suspect caused the additional friction/pressure increase, as with the NiB coating on the Brownell's carrier that I referenced. The carrier key is bolted to the carrier and, if the carrier experiences any additional resistance in movement, a pressure change will occur, and the key would been the first recipient of that pressure change as the interface between the gas exiting the gas tube and entering the carrier key. A very slight/light wet sanding of the carrier coating is what returned my BCG to full functionality, as I stated above.

You were looking for possibilities, right, or are you just looking for some other fault other than the applied cerakote and/or BCA quality?

Absolutely and thanks for your input.

Soganofla

Discussion Starter · #13 · 15 d ago

P938365 said:
Everyone who pointed to metallurgical failure is on point. I can say from experience that a blockage will not result in a split gas key tube. Years ago, my motor sergeant was issued a theater-provided M-4 that would not cycle, other than manually. When my armorer sent it to the third-shop for diagnosis, the mindless fools there only gauged the weapon, so they failed to find the problem.

When it came back with the problem unsolved, my armorer brought the weapon to me, and one of the first things I did was run a pipe cleaner into the gas key, at which point I discovered a plug of q-tip wedged in there. It was in there hard enough that I had to use a sheet metal screw to bore into the mass and pull at it with a Gerber tool, while one of my NCOs pulled the bolt carrier from the other side. We basically had to tear chunks from the mass incrementally before we got it to the point that the rest of it could finally be fully extracted.

Despite that blockage, the gas key's tube did not split. It only resulted in a failure to cycle at all.

Good to know. Thanks.

Reactions: 1

Devereaux

· #14 · 15 d ago

This is aimed kind of at @sigamore.

I do not claim any real understanding of the factors of an AR besides simple functioning. So. In my mind, the BCG rides on the rails, top and bottom, on the front end of the BCG. The rest of the BCG is free of contact with the upper (trigger housing contacts excepted). ?Are you then saying that coating can be thick enough to change the points of contact of the BCG with the upper, or just that it can increase the rail thickness causing more friction, so slowing speed.

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Twister

· #15 · 15 d ago

BCA quality control is questionable at best. I also had problems with a bcg from them that turned out to be an out of spec gas key. @Soganofla definitely not your fault.

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Soganofla

Discussion Starter · #16 · 15 d ago

Twister said:
BCA quality control is questionable at best. I also had problems with a bcg from them that turned out to be an out of spec gas key. @Soganofla definitely not your fault.

Thanks - that's good to know. As mentioned in a previous comment I found several problems with the upper build - but none of them had anything to do with bolt function so I left these problems out of this particular conversation so as not to confuse the issue.

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sigamore

· #17 · 15 d ago

Devereaux said:
This is aimed kind of at @sigamore.

I do not claim any real understanding of the factors of an AR besides simple functioning. So. In my mind, the BCG rides on the rails, top and bottom, on the front end of the BCG. The rest of the BCG is free of contact with the upper (trigger housing contacts excepted). ?Are you then saying that coating can be thick enough to change the points of contact of the BCG with the upper, or just that it can increase the rail thickness causing more friction, so slowing speed.

As you can see in my photo below, which happened to be handy since I am in the middle of removing braces from various AR pistols, the BCG rides on the lower rails of the upper receiver. In the pictured Ballistic Advantage nitride BCG mounted in an Enhanced Ballistic Advantage upper receiver, that is a very much desired tight tolerance and there is very little wiggle between the BCG "slides" and the receiver rails. Now imagine that I add a coating to the carrier, remembering that whatever thickness of coating is applied, it actually doubles the total thickness of the BCG to rail fitment because it is applied all the way around. From my experience, that very thin coating, including a factory NiB coating, can result in additional sliding friction. It has never caused a catastrophic failure for me, such as the split key in the OP's case, but it can impact cycling and, if the quality of any of the components is suspect, it could expose such a defect. Just food for thought based on personal experiences. If the cerakoted BCG was returned to BCA, my guess is that their immediate response would be that the failure was due to the cerakote.

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P938365

· #18 · 15 d ago

sigamore said:
It has never caused a catastrophic failure for me, such as the split key in the OP's case, but it can impact cycling and, if the quality of any of the components is suspect, it could expose such a defect. Just food for thought based on personal experiences. If the cerakoted BCG was returned to BCA, my guess is that their immediate response would be that the failure was due to the cerakote.
View attachment 527223

The bolded portion of your statement is the most salient. However, if an outright blockage of the gas key can't cause a proper one to rupture, then some increased friction between the bolt carrier and receiver shouldn't rupture it.

I've seen a lot of new AR owners at the range who didn't know to lube their BCGs when they went out to shoot them for the first time, and they experienced some horrific stoppages resulting from the BCGs only traveling about halfway to the rear because of the friction.

If a manufacturer tried to attribute cerakoting-induced friction to the failure, then the manufacturer would have to explain why poorly lubed new ARs didn't all have similar gas key tube ruptures, too.

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sigamore

· #19 · 15 d ago

P938365 said:
If a manufacturer tried to attribute cerakoting-induced friction to the failure, then the manufacturer would have to explain why poorly lubed new ARs didn't all have similar gas key tube ruptures, too.

Not arguing about the real reasons for the failure but, in m opinion, the manufacturer would look to pass the blame elsewhere and seeing that cerakoted BCG would probably be an item of focused attention.

@Devereaux - you asked about "points of contact" for the BCG. As a point of reference, this video shows significant areas of impact and wear areas...

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P938365

· #20 · 15 d ago

sigamore said:
Not arguing about the real reasons for the failure but, in m opinion, the manufacturer would look to pass the blame elsewhere and seeing that cerakoted BCG would probably be an item of focused attention.

Roger that. I already figured you were wargaming the scenario, so I just added a counterargument that could be issued if they try to go that route.

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