AR 7.62x39 stopped cycling

[Garandman3]

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.

[stagpanther]

Hmmmm...I personally would have tried messing with the magazine first before doing all that invasive surgery; especially since you seem to indicate the failure happened at a particular cartridge position. The 7.62 x 39 is a bit of a tricky one in an AR magazine. I have the "low end" PSA AR47 upper and it uses the "regular" AR style magazine--I think at some point they switched to AK magazine compatibility; is that what you have?

[Shadow9mm]

Check the gas rings, quick easy fix. Seems like you are trying to add more gas to make it run, but if the gas rings are shot and not holding pressure it is only a temporary fix. As mentioned it could be a weak magazine spring with the failures to feed near the end of the magazine, if you out 5k rounds through 5 mags they may need new springs. As always only change 1 things at a time. If you change multiples it is really hard to tell if what you are doing is helping or not.

[smee78]

"but it still failed the last round in the magazine test" is this with all 5 mags or are you only using one mag? I would choose one mag and put a new spring in it and only use it from now until you get the problem sorted out. Also when you say "it stopped cycling" and "After all this it still did not cycle properly" are you saying it wont shoot at all or is it short stroking or??? Over gassing the gun will not be a fix, is your gas block clean and did you perform proper cleaning during the 5000 rounds you shot? Was there any escaped gas signs where the gas block was installed? Did you check your gas tube for cracks or damage?

[FrankenMauser]

Sounds like you ran into a maintenance and magazine issue, tried to solve it by over-gassing the thing, and now it may be impossible to isolate the variables without testing individual parts in another receiver.

Try different ammo and another magazine.

When you only have a feeding issue with one round in a magazine, in the same position every time, it is almost never the firearm. It is the magazine, 98% of the time. And usually influenced by ammunition, the rest of the time.

If the second to last round feeding issue was the only problem, you probably did a bunch of unnecessary work because of a weak magazine spring.

I know the popular thing to do is just start throwing gimmicky buffers and springs, and special gas tubes, and adjustable gas block, and 'special' BCGs are everything in today's world. But most ARs, including freaks running odd cartridges, don't need that garbage. The vast majority of AR malfunctions are caused by magazines and crap ammo.
(And much of the rest by bad and/or gimmicky parts.)

[brasscollector]

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.

Take your x39 bolt and install it into another bolt carrier. Use your 223 carrier if need be, just to test.

If you aren't loading your own x39 ammo you are running some DIRTY stuff through your gun. My guess is that your gas system is choked up with crud. You should probably verify with compressed air whether or not you are actually flowing air through the gas key. And I'm not talking about the upper tube part that the gas tube slides into. My concern would be the rear end of the key that is near impossible to get any sort of cleaning device into.

[Garandman3]

StagPanther
I probably should not have mentioned the failure to feed the second to last round. This happened with only one mag and was resolved by bending the lips outward.

[Garandman3]

Shadow9mm

In the OP "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."

[DnPRK]

Describe what you mean by "failure to cycle".
Failure to ignite the primer?
Failure to extract?
Failure to eject?
Short stroke/failure to pick up the next round?
Bullet nose jams high/low and doesn't enter the chamber?
Bolt carrier group fails to go completely into battery?

Each of the above is a "failure to cycle", but have different root causes.

[Skans]

My guess is, without examining it, is that you have a problem with the bolt, gas key or gas tube/block.

[10-96]

Describe what you mean by "failure to cycle".

DnPRK nailed it. We can only throw darts at wild guesses unless we know what is and what isn't going on. Also, tell us about how aggressively you clean the chamber, has anything scrapped it to the point of headspace issues? An out of spec chamber/shoulder/neck area is often over looked and will prevent extraction. Slip us some details Garandman3.

[Bartholomew Roberts]

Sounds like magical gas gnomes have moved into your upper and are siphoning off some of your gases to power their tiny forges. Do you know any druids?

[Garandman3]

In the OP I stated that it failed the last round in the magazine test and that the spent cases were ejecting in the 3 to 4:30 sector. From those two you can infer that the round was chambered, fired, extracted and ejected but the bolt failed to be caught by the bolt catch. If the BCG was not pushed back far enough to be held open by the bolt catch then it was not pushed back far enough to strip the next round (if there had been one) out of the magazine.

[stagpanther]

If the BCG was not pushed back far enough to be held open by the bolt catch then it was not pushed back far enough to strip the next round (if there had been one) out of the magazine.

Fair enough--but then how do you explain it worked on all but one cartridge?

[10-96]

OK, as much as you're opened up the gas system- that ought to be out of the question. Maybe the BCG is going back far enough, and the bolt catch is not catching on the bolt carrier? Are there any shiney spots on top of the catch that shows the carrier is passing over the top of it instead of catching? You can cut a small piece of aluminum out of a can and glue it to a magazines catch ledge to see if it starts catching. If it does, pry off the aluminum and replace the catch.

[stagpanther]

7.62 x 39 has a tapered case, which means that it can introduce a pitch in the cartridge's alignment in a conventional magazine (one reason AK magazines are curved). Depending on the magazine's anti-tilt efficiency, that angling off a cartridge might be causing problems with the carrier.

Magazines are cheap (relative to things like the barrel and carrier) and I would be inclined to mess with it to see if I could figure out what is going on. I like 10-96's thinking--another thing I might take a look at is how the magazine positions in the lower relative to the magazine catch, I have have seen on occasion magazines with the slot for the magazine catch be cut just a smidgen low or high which is enough to throw the reliable cycling functioning off. Might want to disassemble, clean and reassemble the magazine catch components (making sure they are using the right components properly positioned) in the lower as well. When I have feed issues I like to feed in slow motion a cartridge to see if I can isolate what might be happening. I'd disassemble the magazine clean it, maybe stretch out the spring to increase the compression tension to see if that helps. Sometimes modifying the feed lips on a magazine is needed to get the cartridge aligned properly. Just throwing some ideas out there.

[Garandman3]

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.

[10-96]

Barrels really aren't that hard. In my limited knowledge of metallurgy (none), if I had to pick a description for barrel material, I would have to say it's "gummy". If a barrel is too hard, it seems it would run the risk of shattering, which they very seldom do. Too soft, they just wear out too fast. In my line of thinking after seeing barrels split and curl, but yet not fragment, and maintain the ability to be smoothly machined- "gummy" isn't right, but I can't think of a better word right now.

Anyway, it's a whole heck of a lot harder to drill into most bolt action receivers than any barrel I've ever seen.

[Metal god]

Ok you sucked me in , how did changing the barrel fix the problem of cycling ? I’m thinking it could only be a gas port issue if it’s the barrel thats the problem ??? Which if clogged cleaning it out is the answer or if erodded using an adjustable gas block ??? I have to admit changing the barrel would not be at the top of my list of fixing cycling issues and may have never crossed my mine unless I was looking for a reason to change it .

Hoping this becomes a good learning experience for me .