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Posted: 1/27/2014 8:41:15 AM EDT
My AR has an issue were after about two mags (60rds) it'll start misfeeding were the bolt will scrape along the top of the next round. I'll clear that and after around 20 more rds it'll get to the point were the BCG doesn't even pull back at all after firing. Pulling on the charger will eject the round and reload the next one but after each shot I have to manually charge and fire.

It's well lubed and cleaned with about a total of 600 rds through it. The gas key is tight and I've tried different magazines. The most baffling thing about this to me is that the weapon fires fine when relatively cold but as it heats up the conditions get worse to the point that the BCG doesn't move at all when fired.

Link Posted: 1/27/2014 9:19:07 AM EDT

I've seen cam pins out of spec that would cause issues when the gun heats up, but never a complete lack of unlock. Is it physically hard/more resistent to cycle by hand when its hot?
Link Posted: 1/27/2014 9:27:04 AM EDT
Just a quick question. What kind of rifle is it and are the receivers aluminum or polymer?
Link Posted: 1/27/2014 9:28:13 AM EDT
No, I didn't feel an increase in resistance throughout. And I couldn't find any obvious signs of wear and it was still well lubed (slip 2000) after it degrades into "single shot mode".

The cam pin, is that the cotter pin on the side of the BCG holding the firing pin in place? A buddy of mine suggested that might be the problem and to check it for loseness but it feels snug and has resistance when pulling it out.
Link Posted: 1/27/2014 9:31:06 AM EDT

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Originally Posted By RDHockey:


No, I didn't feel an increase in resistance throughout. And I couldn't find any obvious signs of wear and it was still well lubed (slip 2000) after it degrades into "single shot mode".



The cam pin, is that the cotter pin on the side of the BCG holding the firing pin in place? A buddy of mine suggested that might be the problem and to check it for loseness but it feels snug and has resistance when pulling it out.
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No the cam pin is the pin that goes through the bolt and "cams" in the slot on top of the carrier.

 
Link Posted: 1/27/2014 9:36:03 AM EDT
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Originally Posted By BB:
No the cam pin is the pin that goes through the bolt and "cams" in the slot on top of the carrier.  
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Originally Posted By BB:
Originally Posted By RDHockey:
No, I didn't feel an increase in resistance throughout. And I couldn't find any obvious signs of wear and it was still well lubed (slip 2000) after it degrades into "single shot mode".

The cam pin, is that the cotter pin on the side of the BCG holding the firing pin in place? A buddy of mine suggested that might be the problem and to check it for loseness but it feels snug and has resistance when pulling it out.
No the cam pin is the pin that goes through the bolt and "cams" in the slot on top of the carrier.  


Ah, I'm not sure how to know if it's out of spec, but the top square portion does spin awfully easily, to me at least, like it's loose. I don't have another bcg to compare it to.
Link Posted: 1/27/2014 9:47:51 AM EDT
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Originally Posted By RDHockey:


Ah, I'm not sure how to know if it's out of spec, but the top square portion does spin awfully easily, to me at least, like it's loose. I don't have another bcg to compare it to.
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Originally Posted By RDHockey:
Originally Posted By BB:
Originally Posted By RDHockey:
No, I didn't feel an increase in resistance throughout. And I couldn't find any obvious signs of wear and it was still well lubed (slip 2000) after it degrades into "single shot mode".

The cam pin, is that the cotter pin on the side of the BCG holding the firing pin in place? A buddy of mine suggested that might be the problem and to check it for loseness but it feels snug and has resistance when pulling it out.
No the cam pin is the pin that goes through the bolt and "cams" in the slot on top of the carrier.  


Ah, I'm not sure how to know if it's out of spec, but the top square portion does spin awfully easily, to me at least, like it's loose. I don't have another bcg to compare it to.


But reading up on what the cam pin does I don't think this is the problem. After the rifle heats up the bolt doesn't move back at all when fired, the whole bcg stays in place. If I then pull back on the charger the bolt comes back and the cartridge ejects normally. It appears to be a gas related issue
Link Posted: 1/27/2014 9:57:49 AM EDT
Possibly worn gas rings...
Link Posted: 1/27/2014 10:11:03 AM EDT
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Originally Posted By AlabamaPaul:
Possibly worn gas rings...
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Hmm, I did the stand it on end test and the bolt stayed in place. As I understand it that means that the rings are okay. I'll break it apart and visually verify them as well. Thank you
Link Posted: 1/27/2014 10:31:55 AM EDT
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Originally Posted By RDHockey:


Hmm, I did the stand it on end test and the bolt stayed in place. As I understand it that means that the rings are okay. I'll break it apart and visually verify them as well. Thank you
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Originally Posted By RDHockey:
Originally Posted By AlabamaPaul:
Possibly worn gas rings...


Hmm, I did the stand it on end test and the bolt stayed in place. As I understand it that means that the rings are okay. I'll break it apart and visually verify them as well. Thank you

Broken rings can make it bind and pass the test as well.
Link Posted: 1/27/2014 10:34:23 AM EDT
Out of spec barrel chamber?  What type of rifle or upper is it?
Link Posted: 1/27/2014 11:54:13 AM EDT
its in the gas system



usually a gas block can expand, the gas key might leak a little, or gas rings lose too much.





I would check the gas key, see if one of the screws has broken (only can tell if it twist or not)

Gas block check the set screws if it has them, clean the block and look for leaks,



do the gas ring check


Link Posted: 1/27/2014 12:37:49 PM EDT
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Originally Posted By LIL-COMMANDO:
Out of spec barrel chamber?  What type of rifle or upper is it?
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Lower is a S&W M&P 15 alumnium. Upper is a JSE Surplus 16" HBAR


Link Posted: 1/27/2014 6:57:29 PM EDT
I think I found the problem. I'm new to the AR platform and didn't know how to, or that I should, remove the bolt from the BCG. The bolt was sticking when I pushed it in and out with my fingers. Once I pulled the bolt out, scrubbed it and the interior of the carrier (which both had build up), then aligned, or more correctly misaligned the O rings, the bolt operates smoothly, it'll stand on end without dropping but it has a nice smooth movement instead of catching. I'm confident that the weapon will fire properly now.

Thanks to BB, drop11, TaylorWSO for your help
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