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AR-15 won't cycle

Discussion in 'Gunsmithing and Repairs' started by Keeperfaith, Dec 3, 2012.

  1. Keeperfaith

    Keeperfaith Well-Known Member

    I built my own AR. The gas block is a DPMS railed gas block which utilizes set screws to keep it in place. The first time I shot the rifle it functioned flawlessly. The next time I shot it the bolt never cycled back to eject the spent case. I noticed the gas block had came loose so I knew there was no gas coming through. Ok no problem, I went home, put the gas block in place and tightened the set screws w/ lock tite.

    Went to shot it today and AGAIN, it won't cycle but The gas block is tight.

    Any suggestions?


    PS: the first time I shot it, I only shot about 30-60 rounds, cleaned it thoroughly and put it away.
  2. Jim K

    Jim K Well-Known Member

    I hope you didn't use the wrong kind of Loctite, because I am going to suggest you remove the gas block and make sure it and the barrel underneath are clean and that the gas port is clean and free. Then reattach the gas block in the right place. Before you reassemble the rifle, make sure the gas port and block are clear by just blowing through them.

  3. Powderman

    Powderman Well-Known Member

    Make sure that the gas block has been properly indexed. Use a good torque wrench--torque to 25-30 lb/in. (inch pounds). I would clean the threads prior to use.

    Check your carrier as well; are the carrier key screws staked properly? The staking/swaging should actually deform the bolts slightly.
  4. TITAN308

    TITAN308 Well-Known Member

    Not to sound silly - but even if the gas block is "tight" you need to make sure that the whole thing is align properly to the opening in the barrel.
  5. Keeperfaith

    Keeperfaith Well-Known Member

    Ok I took it apart yesterday, cleaned the gas tube, barrel port and made sure there were no obstructions. Put it back together and fired it again. It functioned fine the first 5-10 rounds and then it was back to no bolt cycling all the way. The bolt cycled enough to reset the trigger and eject the case but not load a new round. It also fired and had no bolt travel, no case eject and no reset trigger. I also got about 4-6 malfunctions (new round caught in chamber.

    Now I will admit I had a difficult time indexing the barrel nut w/ proper torque and keeping the gas tube hole aligned. There is a slight tension on the gas tube either left or right when it's aligned with the gas block and barrel nut. Could this be my problem?
    Also there's some slight wear in the gas tube roll pin hole now that I've taken it off twice to inspect it. Could this have added to my issues?

  6. mtrmn

    mtrmn Well-Known Member

    Yes it could have something to do with your problem. The gas tube needs to extend into the upper receiver in the center of the channel for your gas key.
    Take the upper off the lower. Remove the BCG and disassemble it. Turn the upper half upside down and install JUST THE BOLT CARRIER without the bolt. Gently slide the bolt carrier forward. Is there any resistance felt when the gas key meets the gas tube? You should only feel a very slight bump, if that. If there is very much resistance at all, the gas tube is not aligned properly. Depending on how bad it is out, it could damage the end of both the tube and the key. VERY Slightly bending the gas tube in the upper will remedy this if neither component has been damaged due to collisions.

    The gas key must be tight on the bolt carrier and the screws staked solidly to keep them tight.

    If you can manage to get some compressed air or something similar into the barrel or gas tube, you can use that to determine the best position for the gas block to fully match up with the barrel. Find the position with the greatest amount of airflow.

    Also, if you're using weak ammo (wolf etc) for the testing, it may also be contributing to the short-stroking. Too heavy a buffer/spring will make this worse. I don't know if it's even possible, but if you have a carbine buffer tube and somehow got a rifle buffer in there that would surely mess up the operation.

    What do the gas rings on the bolt look like?
    Last edited: Dec 5, 2012
  7. hentown

    hentown Well-Known Member

    Sounds more like the barrel nut is just a little off and the tube is touching one side of the hole in the barrel nut.
  8. mtrmn

    mtrmn Well-Known Member

    Also use plenty of lube-run it wet
  9. unclecrazy

    unclecrazy New Member

    Have you checked the GAS rings on your bolt ? The gaps may be lined up together. Stagger the Gaps if needed.
  10. leftym4

    leftym4 Active Member

    My bet is as others said in the indexing of the gas block and alignment of the gas tube, i have seen this case on other forums and it has just about always been the issue. the gas port hole in the block must align with gas port hole in the barrel enough to get enough gas to cycle.

    I doubt the problem is in the rings unless they are worn and not staggered correctly in non staggered rings will run fine until worn out and the combo of the two.

    gas leaks in the key from loose screws can cause this problem

    Also are you running the correct buffer for this set up? Lots of extra heavy buffers on the market if your running a H2 or heavier this can cause this problem.

    did you lube the buffer itself? helps to have some white lithium greese on it but that alone would not cause this just helps to have.

    are you using brass cased new ammo ? sometimes steel cased or poorly reloaded or weak ammo can cause this

    did you try different magazines ?

    if its none of the above you may have a out of spec part either a gas port in the barrel or block misaligned . I out of spec gas key I had one of these once on a brand new factory build grendle upper i swopped keys and the problem was fixed .

    other factory problems it could be misaligned locking lugs on the barrel
    out of spec camber i have heard of this with happening on budget barrels i wont mention brands but manufacturers will usually replace them.
  11. rcmodel

    rcmodel Member in memoriam

    Which matters not anyway.

    If everything else in the gas system is right, staggered or lined up makes absolutely no difference at all.

    Gas rings are constantly moving, and get in alignment and out of alignment by themselves all the time.

  12. Keeperfaith

    Keeperfaith Well-Known Member

    Hey guys I appreciate all the posts. I really do think its the alignment of the barrel nut. Its forcing the tube to not be properly aligned w/ the gas key. The bolt will fit if I give it a push (into the gas tube in the upper) but i'm pretty sure its the problem.

    Thanks for the help.

    I'm waiting on my AR tools (i lent em out to my bud so he could build his AR).

    I'll let yall know the results when I tweak my barrel nut.


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