7.62x39 AR stopped cycling

In April 2017 I built a 7.62x39 AR using a Palmetto State Arsenal 18" 7.62x39 mid-length complete upper, a Polymer80 lower and a lower parts kit. I purchased 5 C Products 10 round magazines, a scope and 200 rounds of Wolf Military Classic ammo. The rifle functioned beautifully right from the start. Since then I have fired an additional 5000 rounds of the same ammo and then it stopped cycling. Prior to it failing to cycle it would occasionally fail to feed the second to last round in the magazine but otherwise the gun functioned fine with good accuracy. I also have AR's in 5.56 and 350 Legend. I swapped lowers from them to the 7.62x39 upper but it still failed the last round in the magazine test. The 5.56 and 350 Legend uppers functioned fine with the 7.62x39 lower. The spent cases mostly ejected in the 3 to 4:30 sector. I installed an H1 buffer and new flat wire spring without success. Convinced that the problem lay in the upper, I removed the gas block and gas tube, plugged the hole in the gas block and filled the tube with KG Systems Carbon Remover and let it soak for 30 minutes. The solvent was clear when the tube was filled but was deep blue with tiny black particles when drained. I drilled out the barrel gas port from 1/8" to 9/64". The difference in the OD of the gas tube and the ID of the gas key is 0.003"/ 0.004". The hole in the bolt carrier was slightly blocked by deposits. I drilled out the slanted hole in the gas key with a 1/8" bit. The bolt rings passed the test in which the extended bolt supports the BCG. It didn't collapse even when a 12' tape measure was placed on top. After all this it still did not cycle properly. I even removed all the weights from the buffer with no success. I don't know which part of the upper is at fault and with this present hysteria it is hard to find parts in stock.

Try cleaning the chamber real good with a heavy bronze or stainless steel brush. Report back.

Is it failing to feed a round out of the magazine into the chamber? I think I read you swapped uppers and lowers to test each other. I can tell you from MY EXPERIENCE, it was the C Products Defense 10 round magazines. No joke. I had misfeed after misfeed. Tried 2 different lowers, 2 different 7.62x39 uppers. Was the 10rd mags. The 10rd mags use a 5.56 follower, unlike the 20+ mags, which use a 7.62x39 follower. I even had C Products replace the mags with newer improved ones, and they still failed. Where are you located in MD? Generally speaking.

Check the gas key on your BCG to see if it's fully or parially plugged or if the screws have sheared.

inkd said:

Check the gas key on your BCG to see if it's fully or parially plugged or if the screws have sheared.

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In the OP I stated: "The hole in the bolt carrier was slightly blocked by deposits. I drilled out the slanted hole in the gas key with a 1/8" bit." I had to remove the gas key to observe the deposits in the bolt carrier and to drill out the slanted hole.

John from MD said:

Try cleaning the chamber real good with a heavy bronze or stainless steel brush. Report back.

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I may have done this in the initial efforts to resolve the malfunction. In any case, I took your advice and when the weather gets better I'll see if it makes a difference.

tony7815 said:

In the OP I stated: "The hole in the bolt carrier was slightly blocked by deposits. I drilled out the slanted hole in the gas key with a 1/8" bit." I had to remove the gas key to observe the deposits in the bolt carrier and to drill out the slanted hole.

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When you say it stopped cycling, at what part of the sequence is it failing? Extraction, ejection or feeding?

Did you check the reverse torque of the gas key screws when you removed the key? When you reinstalled the gas key, did you torque and stake the gas key screws? Did you use any gasket sealer between the gas key and bolt carrier?

Before you removed the gas block, did you check to see if it was aligned properly? Is it a set screw type, if so, were the screws tight?

Are you sure the gas block is properly aligned now that it's been reinstalled?

What does the gas tube look like on the end that mates up with the gas key? Have you taken the bolt out and dropped the carrier into the upper to see if it goes into the gas key with no resistance?

inkd said:

When you say it stopped cycling, at what part of the sequence is it failing? Extraction, ejection or feeding?

Did you check the reverse torque of the gas key screws when you removed the key? When you reinstalled the gas key, did you torque and stake the gas key screws? Did you use any gasket sealer between the gas key and bolt carrier?

Before you removed the gas block, did you check to see if it was aligned properly? Is it a set screw type, if so, were the screws tight?

Are you sure the gas block is properly aligned now that it's been reinstalled?

What does the gas tube look like on the end that mates up with the gas key? Have you taken the bolt out and dropped the carrier into the upper to see if it goes into the gas key with no resistance?

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This is all sound advice....a SOTAR grad, hmmmm????

Might try looking at the front of the receiver’s threaded extension hole to see if the carrier is striking the receiver. Also, is the bolt hitting any of the receiver small parts?

3paul10 said:

This is all sound advice....a SOTAR grad, hmmmm????

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I may have attended a class or three

Any resolution to this issue?

Sirex said:

Is it failing to feed a round out of the magazine into the chamber? I think I read you swapped uppers and lowers to test each other. I can tell you from MY EXPERIENCE, it was the C Products Defense 10 round magazines. No joke. I had misfeed after misfeed. Tried 2 different lowers, 2 different 7.62x39 uppers. Was the 10rd mags. The 10rd mags use a 5.56 follower, unlike the 20+ mags, which use a 7.62x39 follower. I even had C Products replace the mags with newer improved ones, and they still failed. Where are you located in MD? Generally speaking.

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I say this^^^
If the BCG was reassembled properly(gas key resurfaced, sealed, and screws properly torqued), then he could try an H2 buffer to maybe slow the bolt down a little, but I still think the problem could be in the mags. Springs wear out, and so can poorly designed followers. It sounds as though the mags were the only thing he didn't swap out.


ETA: I seen a couple SOTAR youtubes...

I replaced the barrel and the rifle is functioning just fine. The ease in which I was able to drill the barrel caused me to doubt the hardness of the barrel.
Thanks for all the input.

tony7815 said:

I replaced the barrel and the rifle is functioning just fine. The ease in which I was able to drill the barrel caused me to doubt the hardness of the barrel.
Thanks for all the input.

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Thanks for the update.