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Thread: Bolt not locking to the rear when mag runs dry

  1. #1
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    Bolt not locking to the rear when mag runs dry

    My weapon has had this issue the past couple of times I have gone to the range. Sometimes the bolt locks back on a dry weapon, and sometimes it doesn't. I lube up the BCG and and the buffer assembly generously and then it solves the problem until about 3 or 4 mags later, then it starts happening again. I'm thinking maybe it has something to do with the low profile gas block that I installed to replace the front sight gas block when I got a longer free-float rail, as the weapon didn't have this problem prior to the change. I suppose it might also have something to do with the buffer, as I changed those out some time ago too. But it's an H buffer and I have always been under the impression that that's the best buffer for a 16 inch barrel with a carbine length gas system. Does anyone have any suggestions about what might be causing this malfunction. Any ideas would be very much appreciated, as I know that there are folks on here that know infinitely more than I do about trouble shooting the AR platform. Thanks in advance.

  2. #2
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    Probably is the gas or lack thereof.




    Who did the gas block install? Check to see if its aligned properly.



    H buffer should not be an issue with a carbine.



    Check the gas tube to make sure its installed properly.



    Try locking the bolt to the rear several times with your empty mags. If the mag spring is weak it should do it manually too. I had a mag do this to me last range trip. It lost enough strength it wouldn't push the bolt catch up anymore. I could repeat it cycling the bolt manually.


    But anytime you change something, and then encounter problems that did not exist before 99% of the time its something you changes. Either a out of spec part or a bad install. Start there...

  3. #3
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    Buffer selection is not a simple match up to barrel length and gas system. It depends on the gas port size too.

    Buffer selection may or may not be part of the issue.

  4. #4
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    Where is your ejection?

  5. #5
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    Could be a bent bolt catch roll pin. When I assembled my dd lower, I bent mine slightly, resulting in spotty reliability. Like belmont said, try replicating it manually. Do this with the upper off and an empty mag seated and watch everything.
    Acta Non Verba

  6. #6
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    I was having same problem with my MP-15 with the H buffer. put the carbine back in and had no problems
    Ash Hess

    Government Sales Specialist at Knights Armament Company

    ahess@knightarmco.com

    Senior writer of TC 3-22.9 Rifle and Carbine
    US Army Master Marksmanship Instructor.
    Sionics Weapon Systems AR15 Armorer


  7. #7
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    check for alignment, as belmont said, on the gas block- also check for carbon leakage at the gas block and gas tube. if you're leaking bad enough to short-stroke, you'll probably have a pretty good size deposit of black shit on your barrel and/or on the tube.

    have you checked your gas key? make sure your screws are tight and staked

  8. #8
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    What magazines are you using??

  9. #9
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    Quote Originally Posted by bkb0000 View Post
    check for alignment, as belmont said, on the gas block- also check for carbon leakage at the gas block and gas tube. if you're leaking bad enough to short-stroke, you'll probably have a pretty good size deposit of black shit on your barrel and/or on the tube.

    have you checked your gas key? make sure your screws are tight and staked
    I'll check it out ASAP. I'll also check to see where my rounds are ejecting. Thanks guys.

  10. #10
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    Quote Originally Posted by Thomas M-4 View Post
    What magazines are you using??
    All PMAGs

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