AR malfunction question- conclusion

[walnutred]

My new son bought an AR at a gunshow which looks to be a build using mostly Aero Precision parts. The rifle looked to be unfired and when we got to the range found out why. The rifle would not cycle the bolt. Gas block seemed slightly out of adjustment but readjusting the block didn't help.

We put a known good bolt carrier group into the AP rifle and everything functioned like it should. We put the BCG from the AP rifle in another rifle and that rifle would not cycle the bolt when fired. The rifle fires but the BCG only goes back about 3/8".

So we started swamping BCG parts around trying to pin down the part causing the malfunction. The only part that would not function in any rifle was the bolt carrier from the AP rifle. The gas key seemed tight and properly staked. What can go wrong on a bolt carrier? I've never seen this problem before and it's frustrating.

Gas key port was clear. New son took the rifle to a local to him gunsmith for evaluation. The gunsmith told him it was probably a headspace issue, it would cost $35 to fully diagnose plus repair. Gunsmith didn't use a go-no go gauge. He decided that since the bolt worked in a different bolt carrier he would simply replace the bolt carrier. We examined the fired cases and didn't see any indication of excessive head space. Thanks for the suggestions.

 

[29aholic]

If some ocd person lined up the gaps in the gas rings on the bolt it won't cycle properly

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[bulletslap]

Does the bolt move in the bolt carrier like it should ?

Many years ago, pre-internet, I bought a bolt carrier group that had a bolt carrier that was “bored crooked” and it restricted the movement of the bolt head and rings enough to cause malfunctions.

 

[walnutred]

If some ocd person lined up the gaps in the gas rings on the bolt it won't cycle properly

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I checked that and the bolts are one of the parts we swapped around.

 

[walnutred]

Does the bolt move in the bolt carrier like it should ?

Many years ago, pre-internet, I bought a bolt carrier group that had a bolt carrier that was “bored crooked” and it restricted the movement of the bolt head and rings enough to cause malfunctions.

The bolt seemed stiff to me but I attributed that to lack of use. I may have been wrong.

 

[Eric300]

What type of gas system (carbine/mid-length/ rifle), and I'm guessing either carbine or mid-length, but what weight buffer spring and what weight buffer? If everything else seems to be in order and functioning properly, check your buffering system. Just a thought.

 

[jachd17]

Is there an obstruction in the gas key itself? Sounds silly but I have seen a broken off Q tip in a gas key once.

 

[oink]

No suggestions I just want to follow this thread.

 

[Benton Quest]

If some ocd person lined up the gaps in the gas rings on the bolt it won't cycle properly

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That is actually a myth. Try it sometime. It makes no difference.

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[29aholic]

That is actually a myth. Try it sometime. It makes no difference.

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BS I have

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[Catbacker88]

BS I have

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Well, a stopped clock is right twice a day, too. This aligning rings thing is in fact a myth and, if you're seriously suggesting it's the problem in this particular case, I absolutely guarantee you're wrong.

 

[walnutred]

Thanks for the suggestions. I don't know why I didn't think of an obstructed gas key. I'll let him know to check that and post results. There isn't much to a bolt carrier which is why it has me stumped. Had the known good BCG not functioned the buffer and spring would have been my next place to look. IT is a rifle length gas tube on a 20" barrel.

 

[Fishinfool]

If the new BC did not work in two rifles, but the new rifle did work with a known good BC, I would tend to think the problem is with the new BC. You mention the new BC seemed stiff in function, which leads me to further think bad BC.

You could try to get it working, but my frustration level being what it is sometimes, I would probably get a new BC group from PSA when they are on sale for under a hundred bucks, and call it a day. Sometimes parts just don't fit right.

The fact the new rifle worked fine with another BC further makes me think it isn't a gas or buffer issue in the new rifle.

R/E the gas rings - That they be properly aligned was made an issue of during my first range visit as a new Fed LEO. Wives tale? Never tested it in actual use, but it might make an interesting comparison some day when I am doing an "AR" day at the range. Since they do rotate freely, I have my doubts it makes any real difference.

Larry

 

[Retired W4]

Well, a stopped clock is right twice a day, too. This aligning rings thing is in fact a myth and, if you're seriously suggesting it's the problem in this particular case, I absolutely guarantee you're wrong.

If it's a myth, the myth came from a reliable source. TM 9-1005-319-10, change 3 (1994) page 36. For the uninitiated that is the US Army Operators manual for the M-16/M4.

I'm not saying the gas ring stagger is the cause of the malfunction.

The dash 10 also says to not interchange bolts from one rifle to another. (I'm paraphrasing)

 

[walnutred]

If it's a myth, the myth came from a reliable source. TM 9-1005-319-10, change 3 (1994) page 36. For the uninitiated that is the US Army Operators manual for the M-16/M4.

I'm not saying the gas ring stagger is the cause of the malfunction.

The dash 10 also says to not interchange bolts from one rifle to another. (I'm paraphrasing)

Yes, we were all taught not to swap bolts. However I saw way too many times when the bolt carrier groups of several rifles were all dissembled and dumped into a parts washer to soak in the arms room. Not saying it's right, just the way it happens.

 

[Retired W4]

Looking at my trusty copy of TM 9-1005-319-23&P (Unit and Direct Support Maintenance Manual)... well, see photos from mythical official Dept. of Army/Air Force, and, oh yea, USMC manuals. It might help the OP. But then it might not.

Notice the old "potato in the tail pipe trick". Oh no that was Eddie Murphy. Same concept.

 

[BB57]

Yes, we were all taught not to swap bolts. However I saw way too many times when the bolt carrier groups of several rifles were all dissembled and dumped into a parts washer to soak in the arms room. Not saying it's right, just the way it happens.

I ended up on an ROTC FTX after a few years in the USMCR. Since I was in the PLC program, and my college didn’t have a Marine ROTC detachment they made me play with the Army ROTC folks. (It was valuable as I learned that the Army taught its officers to think and problem solve differently - and much less independently - than Marine officers.

During the FTX, one of the cadets decided to have all the cadets turn in their bolt/carrier assemblies to be locked up in a few .50 cal ammo cans over night and had already collected them from one of the platoons.

Another cadet had a conniption fit when he saw it happening. He understood head space and was now convinced all of the M16s would have to have their headspace checked before they could be fired.

He wasn’t exactly wrong but he wasn’t entirely right either. M16 parts are parts as long as the parts were manufactured within spec and there isn’t normally enough difference in the bolts and barrel extensions to affect head space to any significant degree.

It’s fairly common knowledge now, but this was before everyone and the their dog was making reverse engineered AR-15 parts and parts kits.

I noted one of the cadre (a Sargent major with three tours in Vietnam) about 50’ away watching how this little drama was going to play out. The two cadets were at an impasse, so I stepped in. I stated since the only ammo on hand were blank rounds, my platoon was just going to keep their M16s assembled since we were in the field, and might be attacked over night since it was an FTX where things like that can happen.

I added that provided the barrel extensions and bolts had been made to spec, headspace won’t be an issue and suggested to the first cadet that he just give the bolts back in reverse order to how he’d stacked them in the box and just not worry about it.

I got a thumbs up from the Sargent major and he walked away.

I didn’t keep up with the cadets involved, but I assume they went on to have successful careers disarming their troops inside the wire in the sandbox.

 

[Benton Quest]

I don't disagree the the whole staggered gas ring thing is in manuals...from almost 30 years ago.

Big green institutional knowledge moves slowly as it is takes a while to turn a battle ship...

But, it is indeed a myth. It has been disproven many times.

An AR will run on one gas ring if it is in spec. One. Not advisable, but it will run.

Yeah. I've tried it. It works.

Old myths die hard....

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[Sandman4delta]

I'm sure that this audience is aware of this but it didn't come up and bears mention if you wish to try and save the BCG or just know for sure.

Rather than the gas pushing on the carrier key, the bolt and the carrier act as a piston and cylinder to spread them and unlock the bolt lugs. The fitment of the bolt in it's hole should feel closely machined but spin freely 360 degrees before you add cam and firing pin.

Not opening and short recoil could indicate it is too tight and resisting movement, or loose/flawed and too much gas bypassing the process.

Hope all goes well!

 

[Retired W4]

I don't disagree the the whole staggered gas ring thing is in manuals...from almost 30 years ago.

Big green institutional knowledge moves slowly as it is takes a while to turn a battle ship...

But, it is indeed a myth. It has been disproven many times.

An AR will run on one gas ring if it is in spec. One. Not advisable, but it will run.

Yeah. I've tried it. It works.

Old myths die hard....

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When in combat, why take a chance? When all else fails, follow the instructions. When you consider who uses M-16s and for what purpose the small details become important. That is why military training is built on small details. when the main purpose of the rifle is sport shooting, who cares?