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Need help diagnosing an AR-15 bolt carrier group problem.

octal

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I've got a brand new bolt carrier group that will eject, but not feed the next round or hold open on an empty magazine. The rifle functions fine with a different BCG, so unless I'm daft, the original BCG must be causing the failures.

As info, when I use the charging handle to manually cycle practice rounds, the BCG will eject and load the next round and hold open on an empty magazine.

Any tips on how to diagnose the BCG? I've never disassembled one before, so what should I look for if I need to?
 
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bamabum

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Look at easy stuff, like lube and the carrier finish. Recently had a similar issue with a Brownells coated NiB 308 BCG. Used some very fine steel wool to lightly "adjust" the finish and it now cycles flawlessly. Next step, if you still have a spare BCG, I would start by swapping the bolts. Easy to do and you are reducing the possible defective part(s) in your troubleshooting. Depending on that outcome, I would then look at the carrier gas key and gas tube alignment, again trying to pinpoint the source. You might need to swap the key, which means possible new gas key screws and the ability to properly stake the new ones. Step by methodical step and you'll figure it out.
 

kenny1773

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Could also be a gas problem
most 'factory' AR15s are over gassed so that would be odd

Do you have an adjustable gas bock? If so, power it up a bit (more gas) and try

If not, is your gas block aligned correctly? again if factory built and built like most off the rack ARs I have seen, it should be over gassed and able to slam any BCG to the rear, but you never know....
 

Rubicon

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I've got a brand new bolt carrier group that will eject, but not feed the next round or hold open on an empty magazine. The rifle functions fine with a different BCG, so unless I'm daft, the original BCG must be causing the failures.

As info, when I use the charging handle to manually cycle practice rounds, the BCG will eject and load the next round and hold open on an empty magazine.

Any tips on how to diagnose the BCG? I've never disassembled one before, so what should I look for if I need to?
Could there be a weight difference between the operational and non operational BCG? Other than friction issues (due to finish, size, whatever), that's the only thing I could see making the difference. Different weights could mean that your buffer spring is too stiff and/or your system is undergassed for the new BCG
 
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octal

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Look at easy stuff, like lube and the carrier finish. Recently had a similar issue with a Brownells coated NiB 308 BCG. Used some very fine steel wool to lightly "adjust" the finish and it now cycles flawlessly. Next step, if you still have a spare BCG, I would start by swapping the bolts. Easy to do and you are reducing the possible defective part(s) in your troubleshooting. Depending on that outcome, I would then look at the carrier gas key and gas tube alignment, again trying to pinpoint the source. You might need to swap the key, which means possible new gas key screws and the ability to properly stake the new ones. Step by methodical step and you'll figure it out.

It was well-lubed and they both have a nitride finish for what it's worth.

I couldn't detect any obvious issues with key/tube alignment and I inserted a spare gas tube into the key and it felt smooth going in and out, but if issues are subtle, that doesn't guarantee me much.

Swapping bolts is a great idea. I'll do that next. Narrowing it down to bolt assy or carrier/key assy would be a great start.
 

octal

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Could also be a gas problem
most 'factory' AR15s are over gassed so that would be odd

Do you have an adjustable gas bock? If so, power it up a bit (more gas) and try

If not, is your gas block aligned correctly? again if factory built and built like most off the rack ARs I have seen, it should be over gassed and able to slam any BCG to the rear, but you never know....

It's an Aero Precision complete upper with a fixed block, minus the BCG, which I bought separately from RGuns. At first I thought was a gas problem but when I swapped the BCG for a PSA one, the gun worked fine.

So there must be something that sets the two BCG's apart that enables one to work but not the other.

The gas block looks OK from what little I can see through the rail which doesn't reveal much. If I end up taking off the rail to look closer, I'll go ahead and replace the block with an adjustable one and go from there.
 

octal

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Could there be a weight difference between the operational and non operational BCG? Other than friction issues (due to finish, size, whatever), that's the only thing I could see making the difference. Different weights could mean that your buffer spring is too stiff and/or your system is undergassed for the new BCG

Weight differences between the two BCGs? I hadn't thought of that. I'll weigh them and see.
 

Tclark

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I've got a brand new bolt carrier group that will eject, but not feed the next round or hold open on an empty magazine. The rifle functions fine with a different BCG, so unless I'm daft, the original BCG must be causing the failures.

As info, when I use the charging handle to manually cycle practice rounds, the BCG will eject and load the next round and hold open on an empty magazine.

Any tips on how to diagnose the BCG? I've never disassembled one before, so what should I look for if I need to?
Hey sir if you are still having trouble with the bcg if you want to bring it down we have a armor here at sentinel firearms in Birmingham
 

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