Bolt Carrier Group Won't Lock Back
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  1. #1
    Junior Member
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    Oct 2016
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    Bolt Carrier Group Won't Lock Back

    Hi Again, all,

    As my earlier thread indicated, my 'new' M&P10 was originally CA compliant, so I've replaced the bullet button with a standard mag release since it is not needed in my state. That's resolved now.

    But it also came with some mods done by a previous owner, who says he never shot it. As I was starting to clean it for the first range trip, I found that the bolt carrier group will not lock back when I press the bolt catch levers.

    My guess - looking at similar problems Google has shown me - is that the Magpul UBR stock installed by the previous owner is too short and/or doesn't have the right buffer and/or spring. Thoughts? Since this was a private purchase, there is no taking it back, so I will need to fix what I have. Should I call Magpul and see what they recommend for the combination of this stock and the M&P10?

    Or may I am jumping to the wrong conclusion, thinking it is the UBR stock?

    Thanks in advance for your help!

  2. #2
    Senior Member
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    Dec 2012
    Missoula, Montana
    5298 times
    Not having the gun to look at and touch. First thing I would look at is, if the bolt is going all the way to the rear. Is the bolt latch the factory piece? If it is not going fully to the rear, use the process of elimination to find out what is blocking its motion. Remove/adjust that blockage and problem solved! Simplistic approach I know but it is really difficult to diagnose a problem without the gun in hand.
    If you can't be who you are, then you don't exist.

  3. #3
    Supporting Member
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    Nov 2011
    Raleigh, NC, USA
    23973 times
    You may be on the right track. Can you draw back the bolt and manually engage the hold open? Does it adequately clear when you do it by hand?

    There is likely a timing issue involved if it physically can clear the bolt hold open.

    The timing is influenced by both the spring and the buffer inside the stock buffer tube. It may not have anything to do with the length of the buffer tube, but everything to do with the mass of the buffer itself. If this was replaced for some reason, try and determine what the stock buffer is on the M&P 10. If they put a buffer intended for the 5.56 in there, and ended up changing the length of the buffer tube itself by putting in the replacement stock, the starting point (returning to factory stock on all the components) is probably the best for diagnosis.

    Look at it this way, if you have to replace it all you still have some new components available for sale, trade or another gun.

    Cheers! Marc

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  5. #4
    Senior Member
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    Jun 2016
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    There are Mil Spec stocks for AR's and Commercial stocks for AR's, and buffers combos to add to the confusion.
    Call Magpul . . . they can direct you to the part# on yours and verify its the correct stock for yours.
    They may be able to tell IF you have correct buffer and spring? "maybe"
    If you have previous owners name . . . can they check records to find what was sold to him (if direct sales)

    I'd suspect he changed things, couldn't get it going correctly, and bailed out of it.



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