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Discussion Starter #1
I did not know where to post this, so I posted it here. I took my AR-15 pistol (300 BO) out today. I think this might even be the first time I have ever fired it. It chambered and fired a round but would not eject. So I came home and thought it was the gas system. I hooked a tube up to it and blew through the tube and there were no restrictions. So I checked the bolt to see if the key was loose, no it was tight. So I took the gas system apart to make sure it was centered on barrel. It was. It ejects manually great. Anyone have any suggestions? Anything I might have overlooked? All ideas appreciated.
 

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I did not know where to post this, so I posted it here. I took my AR-15 pistol (300 BO) out today. I think this might even be the first time I have ever fired it. It chambered and fired a round but would not eject. So I came home and thought it was the gas system. I hooked a tube up to it and blew through the tube and there were no restrictions. So I checked the bolt to see if the key was loose, no it was tight. So I took the gas system apart to make sure it was centered on barrel. It was. It ejects manually great. Anyone have any suggestions? Anything I might have overlooked? All ideas appreciated.
Gas system issues are the most likely problem. Without seeing the gun, it is almost impossible to tell. You hooked a tube up in the receiver and blew...how did you decide the air was moving? It takes two people to detect air going into the barrel. If you put the FSB on yourself, it is easy to misalign the hole in the barrel with the hole in the FSB.

You're saying the bolt didn't move after the round fired? That indicates no gas is making it back to the bolt. Sorry I can't be more specific.

How long is the barrel?
 
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Gas system issues are the most likely problem. Without seeing the gun, it is almost impossible to tell. You hooked a tube up in the receiver and blew...how did you decide the air was moving? It takes two people to detect air going into the barrel. If you put the FSB on yourself, it is easy to misalign the hole in the barrel with the hole in the FSB.

You're saying the bolt didn't move after the round fired? That indicates no gas is making it back to the bolt. Sorry I can't be more specific.

How long is the barrel?
To test the gas system I pulled the bolt, hooked a 1/4" tube to the gas tube and blew through it. Now to check the bolt I checked the key for tightness and also check for leaks. There is a dimple on the barrel to make sure gas block is centered. 8.5" barrel. I am certain it is gas. But I can't figure it out.
 

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Won't eject... Meaning doesn't unlock the bolt? The case doesn't clear the ejection port? What kind of gas block do you have? Adjustable? Is it open enough?

I recently went through hell trying to get my AR upper to work. Turns out it was some sort of tolerance mismatch between upper and lower. Upper worked fine with a different lower--which works fine with its own upper.
 

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Discussion Starter #5
When I say it won't eject, I mean the bolt does not move. Gas block is not adjustable. I ordered the barrel, bolt and block and tube from the same people. A friend and I ordered the exact same set, His works fine, mine not so much.
 

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Commercial rounds or reloads? Cases chamber easily? Bolt unlocks manually with no resistance?

It definitely sounds like gas blockage/leakage. Barrel gas port clear? No soot around gas block or tube going into it? Flushed out gas tube with spray solvent? No blockage in bolt carrier/key? :scratchchin:
 

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Based on what you're describing, it's not getting enough gas, but even some gas would allow the bolt to unlock at least.

I'd try it with different ammo, doing a lock back test (empty mag) to verify it's not ammo related. (that's the 1st place the manufacturer is going to go....)

I'd also pull the gas block and measure the port. I've seen this happen a couple times, that the port was drilled undersized. There's a good reference here:

https://www.tacticalmachining.com/learn/ar-style-rifles/ar-15-gas-port-sizes.html

It could also be 'over sprung" due to a combination of buffer weight and spring, but again that wouldn't explain no bolt movement...
 

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Discussion Starter #8
Commercial rounds or reloads? Cases chamber easily? Bolt unlocks manually with no resistance?

It definitely sounds like gas blockage/leakage. Barrel gas port clear? No soot around gas block or tube going into it? Flushed out gas tube with spray solvent? No blockage in bolt carrier/key? :scratchchin:
Two different commercial and a reload. Cases chamber easily, Unlocks with no resistance. No soot, sprayed with brake cleaner. No blockage with BC/Key.
 

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I had the same thing happened to me one time on a Palmeadow armory with a low-profile gas block. It worked fine and then it locked up because the gas block came loose. It is very hard to open because the casing expands in the chamber and the bolt locks shut. Something is either blocked or misaligned, I almost guarantee it.

That’s why I don’t like home-built guns.
 

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Discussion Starter #10
I had the same thing happened to me one time on a Palmeadow armory with a low-profile gas block. It worked fine and then it locked up because the gas block came loose. It is very hard to open because the casing expands in the chamber and the bolt locks shut. Something is either blocked or misaligned, I almost guarantee it.

That’s why I don’t like home-built guns.
After it fires the case ejects fine manually. I have built all seven of my AR's and never a problem until this one. I am certain it is gas related. I just can't figure out how or why. Everything checks out fine.
 

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Suggestion: remove the bcg, drop the firing pin and pull the bolt. Check bcg gas cylinder for obstructions and bolthead/rings for problems; make sure ring splits are not aligned such that they allow gas bypass.
 

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Start swapping parts with your other home built ARs.
 
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Suggestion: remove the bcg, drop the firing pin and pull the bolt. Check bcg gas cylinder for obstructions and bolthead/rings for problems; make sure ring splits are not aligned such that they allow gas bypass.
I did not even think to go there. But, that's not it either. This is getting frustrating.
 

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Here are my thoughts:

1. No movement of bolt would imply no gas existing the gas tube

2. You state "To test the gas system I pulled the bolt, hooked a 1/4" tube to the gas tube and blew through it.". That would seem to indicate that there is no obstruction.

Assuming the assumptions of #1 and #2 are correct, I think the problem would be a leak. Perhaps the gas block?

I had a very irritating problem with a build. Every time I took it to the range, it would function fine for about one magazine. After that it would not produce enough momentum to fully cycle. After a lot of trial and error I could not find any specific cause. I came to the conclusion that after those first 30 rounds or so, the heat that was generated was causing something in the system to expand and reducing the gas flow. Since I have a pinned flash hider, changing the gas block was not a (cheap) option. So, I replaced the gas tube and have not had an issue since.
 

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When I say it won't eject, I mean the bolt does not move. Gas block is not adjustable. I ordered the barrel, bolt and block and tube from the same people. A friend and I ordered the exact same set, His works fine, mine not so much.
Sorry for the question, but how experienced are you at putting uppers together? I know what you said about the dimple, but there are two things to check.

1) Unless you made sure air moves from the end of the gas tube in the upper receiver all the way through the gas port into the barrel - not an easy thing to check, it is really meaningless. Just blowing air through is useless as air compresses. You could have a misalignment of the FSB or gas block and a tiny hole will allow you to blow through. If there is a misalignment, it is like shutting off the gas system to a varying degree, which finally results in stoppage. In fact, since you say everything is mechanically correct, that is about the only other cause of the bolt not moving. I probably should mention that the bolt cam pin must be assembled properly in the bolt. Make sure it moves freely.

2) I would pull the gas block off and first make sure the hole in the block is the right size. Only the barrel manufacturer can tell you for sure. Were I having that issue, I would buy an adjustable gas block and make very, very sure I have the two ports aligned during assembly - so that the hole above in the adjustable gas block is centered over the gas vent hole in the barrel.

Failing all that, I would take it to a gunsmith - unless you just enjoy hours and hours of tinkering. Good luck.
 
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Gas block holes are generally larger than the barrel gas port. Slight misalignment shouldn't be a factor. I would verify the barrel gas port size. Mine was minimal, and running a 3/32nds drill through it resolved much of the issues I had, although my bolt did operate but didn't feed properly.
 
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