Quantcast

7.62x39 AR stopped cycling

Discussion in 'Gunsmithing and Repairs' started by Garandman3, Jul 9, 2020.

  1. Garandman3

    Garandman3 Member

    Joined:
    May 11, 2016
    Messages:
    7
    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.
     
  2. Blue68f100

    Blue68f100 Member

    Joined:
    May 25, 2011
    Messages:
    6,090
    Location:
    Piney Woods of East Texas
    Drilling things out like you did is not a good idea. Now you will have to add a adj gas block to control the pressure. Ports are sized for a reason. Drilling on the barrel will leave burrs, can damage the barrel's rifling. You may have scrapped your barrel.

    Have you run any air through the gas system to see if it blocked? Plug the barrel and used compressed air to see if any air is coming through the gas tube. Or apply air to the tube and see if it comes out in the barre. Carbon can plug things up hard and make it hard to remove.

    Have you removed all of the carbon build up in the BCG. It takes a special tool to remove this, solvents alone do not work.

    Rings on the bolt should have a 0.005" fit to the bore which is normally 0.5". The OD of the uncompressed rings should measure 0.505"+. But none of this matters if the gas system is not transfer the gas to the BCG.
     
  3. mjsdwash

    mjsdwash Member

    Joined:
    Jan 14, 2011
    Messages:
    1,644
    any scratches in the upper indicating drag? I would have replaced the gas block and tube, they are cheapish. The AR is fairly simple in function. If the barrel port was withing specs for the caliber, and the carrier is okay, theres only the two parts between. Given the high round count with steel core/steel jacket ammo, its easily possible the barrel wore enough that an already marginal gas system no longer has enough pressure to cycle. 5K is enough to flatten rifling, and if it does, burn rates change, and port pressure may drop low enough that the standard gas tube won't have the volume to make it run. I would look hard at the rifling, especially just past the port.

    15/1000" drilling probably didn't screw up the gas system any more than it was.
     
  4. H&R Glock

    H&R Glock Member

    Joined:
    Jul 30, 2010
    Messages:
    1,344
    Location:
    Michigan
    Have you used corrosive ammo??????????????????????
     
    Last edited: Jul 11, 2020
    792mauser likes this.
  5. BreechFace

    BreechFace Member

    Joined:
    Mar 2, 2020
    Messages:
    474
    So, if I'm reading correctly you had cycling issues after 5000 rounds where it wouldn't cycle regardless of magazine capacity at the time, then after changing buffer and spring, cleaning and drilling out ports you now have a rifle that will cycle but will fail to lock back on the last round? All this correct?

    So you've checked the following for alignment and tightness, correct:

    Gas block, you said you removed cleaned and enlarged the gas port
    Gas tube, you soaked with the gas block, have you ensured that it is free of debris by using a pipe cleaner to ensure it will go through without blockage?
    Gas key, see below
    Bolt rings, your test has shown they are in good shape


    Is your gas key tight? You shouldn't be able to loosen the gas key bolts with less than 30in/lbs. of torque. You are on the right track, tracing the gas through the whole process.

    It seems like you have not yet fully inspected the gas key for leaks, I would start there and see if you have carbon flashing out the sides of the mating surface of the key and carrier.

    Do you have another carrier that you can put your bolt into and see if that alleviates any gas key leakage issues?
     
  6. HankC

    HankC Member

    Joined:
    May 8, 2005
    Messages:
    1,274
    Location:
    SW Ohio
    Likely gas leak somewhere, most likely loose gas key. AR gas tube ID is 1/8", gas port does not need to be larger than 1/8".
     
  7. Garandman3

    Garandman3 Member

    Joined:
    May 11, 2016
    Messages:
    7
    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.
     
  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice