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Thread: Bolt not locking back... need your help!

  1. #1
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    Bolt not locking back... need your help!

    I've searched and found several symptoms of bolts not locking back, but none seem to apply. I probably have only 250 rounds of 223/5.56 through my AR, and probably 1200 through my CMMG 22 conversion kit (I'd shoot some 223 through it after the kit to keep the gas tube clean.) Here's what's happening...

    Yesterday I was shooting American Eagle 223, and the first magazine locked back ok. From then on, nothing would lock back... I thought maybe the magazine had a weak spring or bad follower, but none of the other magazines would lock back either. I thought maybe it was weak ammo, so I tried some Winchester 5.56 (which I've never had an issue with) and it wouldn't lock back either. Whenever I'd pull the charging handle back on an empty magazine, it would lock back.

    This made me think maybe it was a gas issue, so I cleaned the gas tube the best I could in case it was short stroking. I took it out today and still had issues. If I manually pushed the bolt catch up when shooting, that would lock it back (so it wasn't the bolt catch, and it wasn't short stroking). I tried a friend's magazine, and a couple others of mine (all are pretty new PMAGS), and none fixed the problem. I then put my upper on his lower (to make sure it wasn't my bolt catch), and it still wouldn't lock back. However, I noticed if I put a little upward pressure on the magazine, that would seem to engage the bolt catch enough to lock it back on an empty magazine.

    Does anyone have any idea what is happening here and why? I've done as much troubleshooting as I know how, but I'm at a loss. Your help is appreciated!

  2. #2
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    What make/model is the rifle in question?
    WTB:
    SA M-7 Classic blonde stock sets
    Bulgy ((10))/((E)) steel mags
    Bulgy T3 takeoff stock sets
    30-caliber magazine clips?

  3. #3
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    Failure to lock back can basically four things.

    1. Bad mag (weak mag spring or bad follower).
    2. Short cycling (not enough gas or weak ammo).
    3. Bad bolt catch.
    4. Overcycling where even a good mag can't keep up with bolt velocity.
    Chief Armorer for Elite Shooting Sports in Manassas VA
    Chief Armorer for Corp Arms (FFL 07-08/SOT 02)

  4. #4
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    Just to be clear, youve had 250 rounds of 223/556 prior without any problems locking back?
    Have you tried another upper on your lower?
    However, I noticed if I put a little upward pressure on the magazine, that would seem to engage the bolt catch enough to lock it back on an empty magazine.
    Was this on your lower or his?

  5. #5
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    The first thing I want to know is who makes the weapon. If it was pieced together then specifically the upper and lower as well as the sum of the parts used to build it.



    Owner/Instructor at Semper Paratus Arms

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  6. #6
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    Quote Originally Posted by DaBears_85 View Post
    What make/model is the rifle in question?
    Double Star lower with DPMS LPK, CMMG 16" midlength upper.


    Quote Originally Posted by Col_Crocs View Post
    Just to be clear, youve had 250 rounds of 223/556 prior without any problems locking back?
    Have you tried another upper on your lower?

    Was this on your lower or his?

    Correct, no previous problems with it locking back. I did not try his upper on my lower (realized later I should have tried it.) When I put upward pressure on the magazine, I had gone back to my lower and hadn't tried it with his.

    Quote Originally Posted by gotm4 View Post
    Failure to lock back can basically four things.

    1. Bad mag (weak mag spring or bad follower).
    2. Short cycling (not enough gas or weak ammo).
    3. Bad bolt catch.
    4. Overcycling where even a good mag can't keep up with bolt velocity.
    1. Tried 4 different magazines total. Two of mine have few rounds through it, one was my friends literally brand new one.
    2. I used hotter 5.56 and still happened... not to mention when I manually engaged the bolt catch it would work, so the bolt carrier was in fact getting back far enough (which seems like it's got enough gas).
    3. Friend's lower/bolt catch wasn't locking back either
    4. This sounds possible. I wish I would have tried his upper on my lower.

  7. #7
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    It's most likely number 2. Could be a lose fitting or lose gas block and or carrier key. Some carrier keys and gas blocks feel tight but in fact leak.

    Quote Originally Posted by SpyderMan2k4 View Post
    Double Star lower with DPMS LPK, CMMG 16" midlength upper.





    Correct, no previous problems with it locking back. I did not try his upper on my lower (realized later I should have tried it.) When I put upward pressure on the magazine, I had gone back to my lower and hadn't tried it with his.



    1. Tried 4 different magazines total. Two of mine have few rounds through it, one was my friends literally brand new one.
    2. I used hotter 5.56 and still happened... not to mention when I manually engaged the bolt catch it would work, so the bolt carrier was in fact getting back far enough (which seems like it's got enough gas).
    3. Friend's lower/bolt catch wasn't locking back either
    4. This sounds possible. I wish I would have tried his upper on my lower.
    Chief Armorer for Elite Shooting Sports in Manassas VA
    Chief Armorer for Corp Arms (FFL 07-08/SOT 02)

  8. #8
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    Quote Originally Posted by gotm4 View Post
    It's most likely number 2. Could be a lose fitting or lose gas block and or carrier key. Some carrier keys and gas blocks feel tight but in fact leak.
    Hmm... any way to check that? I may just have to get ahold of my friend's upper and swap BCGs and see if that affects it, or try my BCG is his upper and see if there's any issue there and try to narrow it down.

  9. #9
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    A few thoughts.

    1. Ask CMMG what size the port is.

    2. Look for leakage around the FSB and where the tube meets it.

    3. Try and see if the carrier keys will move when you try and tighten them.

    4. Try another BCG and also see if you can locate an H or H2 buffer.

    Does it fail to lock back on all magazines? Including one that your friend has? Do they lock back in his weapon?

    Quote Originally Posted by SpyderMan2k4 View Post
    Hmm... any way to check that? I may just have to get ahold of my friend's upper and swap BCGs and see if that affects it, or try my BCG is his upper and see if there's any issue there and try to narrow it down.



    Owner/Instructor at Semper Paratus Arms

    Facebook: https://www.facebook.com/SemperParatusArms/

    Semper Paratus Arms AR15 Armorer Course http://www.semperparatusarms.com/cou...-registration/

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    Master Armorer/R&D at SIONICS Weapon Systems- http://sionicsweaponsystems.com

  10. #10
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    Recently i was changing the buffer to a heavier one in my LMT lower, just to play with the cycle rate and so on. Testing to see if it would indeed lock back on only one round fired at a time, it would not. I had 20 plus new P-mags that i was also testing. It would not lock back. suspected the buffer i was trying, but actually turned out to be the P-mags. I wound up having to file a little excess material off the rear of the mags on about 18 out of 20 i had. They work just fine now. So dont discount the magazines as the problem. Just have to check all things and work thru it...

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