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  • ar10 issues...

    I have an m&p10 that I picked up. I swapped out the stock for a ubr and got a heavy buffer and spring from heavybuffers.com. it's specifically designed to work with 308 rifles and standard ar15 sized stocks.

    The issue I am having is that the bolt will not go back far enough to use the bolt catch when the rifle is all together. If I remove the bcg from the upper and push it into the buffer tube with the buffer and spring in place, it moves back far enough as it should. If I take the buffer and spring out and have everything else together, the charging handle and bolt will move rearward enough to operate properly as well. But if I assemble the rifle as normal it seems like it's bottoming out about half an inch before it should...

    Any ideas on what could be binging or any other things I am not thinking of? Receivers have a nice tight fit with a very minor gap between the upper and lower but the pins are tight and there is no movement between the two.

  • #2
    Sounds like the buffer or spring is too long or too heavy. Try taking just the spring out and trying it by hand. What happens then?
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    • #3
      What was the reason for swapping out the factory buffer and spring in the first place? Did it work with the original parts? If so, there's an easy fix.......

      When I'm teaching a carbine course, by far the most common malfunctions I'll see with AR's is on guns that have a lot of "upgrades".
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      • #4
        It seems to me the buffer and spring work fine. I have removed the spring and put the buffer in the tube and pushed the bcg in by hand and it clears back far enough. Did the same with the spring and it runs all the way back as well. This is with the bcg in my hand and not in the upper receiver. If I take the buffer and spring out of the stock and use the charging handle to move the bcg rearward at that time, it runs back as well. The problem only comes when it is a complete rifle.

        Cmshoot, the reason I swapped out parts wad it came from the factory with a moe a2 length stock and I wanted to run a ubr. The 308 buffer and spring were factory carbine parts that ran a spacer for the rifle length tube but 308 carbine size and ar15 carbine size are different so the 308 carbine buffer and spring were too long to work in the 7" buffer tube as well as the 308 buffer was larger in diameter slightly and would hang up in the gun. Heavybuffers.com has a buffer and spring that are specifically designed for 308 rifles running 7" carbine stocks like the ubr or any other standard ar15 carbine setup.

        Before the heavy buffer kit went it, the factory buffer and spring would bottom out before it moved rearward enough to strip a round from the mag. Now it will but it stops just shy of getting to the bolt catch so last round will hold the bolt on the mag follower right now. My only loss of function at the moment is the bolt catch. I have not been shooting this rifle with this issue. Just functions testing.

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        • #5
          I'm probably not much help..

          I have a similar problem when I run a carbine buffer in my 300BLK. It is just a touch too heavy, it cycles fine, but there is not always enough gas to lock the bolt back. I have also seen those hydraulic damper type in AR 10s do this and it is because they are just a little longer than necessary.
          "Flawed But Repeatable"

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          • #6
            Looks like the buffer is in fact too long. I was measuring with the bolt in the bcg as it would be if locked in the chamber vs extended as if extracting a round... probably going to have to give up on getting a ubr on it as I'm not getting much support from the buffer company even though I've seen other rifles with the same setup and people claiming they are reliable...

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            • #7
              Can you determine how much too long it is? Could that material be removed from the nylon tube end of the buffer? aren't there two different tube lengths available for that set up? isn't there just like .25" difference in them? are the other people using the shorter tube like you are?
              "Flawed But Repeatable"

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