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Thread: Installed MI MLOK rail and gas block on M&P10, accuracy and cycling issues

  1. #11
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    I slight bit of leakage is not unheard of, but it should be very minimal. I have had a loose gas tube before and swapping the tube with another brand/source fixed it. Wasn't causing cycling issues in that case.

  2. #12
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    Quote Originally Posted by Brahmzy View Post
    I slight bit of leakage is not unheard of, but it should be very minimal. I have had a loose gas tube before and swapping the tube with another brand/source fixed it. Wasn't causing cycling issues in that case.
    Unfortunately, the M&P10 uses a unique has tube, and S&W doesn't have any extras available, so I think I'm stuck with this tube regardless. But I will be calling S&W to try and get one on Monday just in case.

  3. #13
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    If you are using the slim MI rail, make sure that your gas block has enough clearance. There is much 'whip' in the barrel, if the gas block makes any contact whatsoever with the inside of the rail during the shot, the harmonics of the barrel are greatly affected and your POI will be all over the place. As far as the cycling issue, did the block move at all while you were shooting? If it were cycling fine, then reliability began to degrade, I would suspect movement (loosened screws?) or a possible blockage that could be a result from a gas port burr being shot off and entering the gas tube etc,..

    Quote Originally Posted by masakari View Post
    So, I recently installed a Midwest Industries MLOK rail and MI gas block on my M&P10. Before the installation, the rifle seemed to function fine but I only fired around 200 rounds. It had no cycling issues and was very accurate.
    Today was my first time out shooting since installation of the rail and gas block, and I had accuracy issues, shooting up to 6 inch groups at 100 yards. I wasn't able to get a good zero, which was my goal today.
    I also began having cycling issues which became worse the more I shot. By about 50 rounds my rifle was completely single shot, and would not strip a round out of the magazine.
    Two types of ammunition were tried, PMC and ZQI. Multiple magazines were used with the same effect.
    I'm fairly well-versed in building ARs, and have never had a problem in the past. This one has me stumped.
    Does anyone have any ideas as to what could be wrong? This rifle was supposed to be done today, but now I'm back to square one. Thanks for your time.

  4. #14
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    It happens. I am also 99% sure that the gas block leakage and misalignment was causing your problem. I may be completely wrong.

    Quote Originally Posted by masakari View Post
    Ok, the problem was two part.

    Firstly, without that .030" gap between the gas block and shoulder, the gas port was very slightly occluded. This, however, wasn't the major cause of the problem.

    I found that gas was leaking out between the tube and gas block. There is a slight wiggle when the two are joined together, and there is gas residue both on the front outside of the gas tube, and inside the handguard.
    I tried putting the gas tube back inside the original gas block and it's a very tight fit. But in this Midwest block, it's fairly loose. This leads me to believe that the gas block itself was machined incorrectly.
    On Monday I'll be calling Midwest. Might get a BCM block at G&R Tactical on my way through Ohio on Tuesday though.
    Anyone else ever have this issue with a gas block?



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  5. #15
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    Quote Originally Posted by masakari View Post
    Unfortunately, the M&P10 uses a unique has tube, and S&W doesn't have any extras available, so I think I'm stuck with this tube regardless. But I will be calling S&W to try and get one on Monday just in case.
    Chances are, the tube is going to be the same diameter and won't change anything. If I recall, the tube on my M&P10 is a bit loose inside the after market gas block I used. Try installing the gas blockb and the original gas tube so the ports line up and see if that works for you. If not, it's the gas block that needs to be replaced, not the tube
    Last edited by MistWolf; 04-02-16 at 04:05.
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  6. #16
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    OK so here's an update:
    I got a new gas block from Midwest industries (also tried hand fitting one from CMMG, felt no looser or tighter) and new gas tube from S&W. Reassembled the rifle, and made sure to have that .030" gap between the block and shoulder. Just shot 32 rounds and while they all fed and fired, the rifle would not lock back on the last round with two types of ammo.
    There is only a VERY slight bit of gas on the gas tube, which from what I read is actually normal anyway.
    So the question is... What the hell is going on? This rifle is seemingly not getting enough gas and I can't explain it.
    The carrier gas key looks and feels tight. Staking is good. I'll keep looking at the bolt and carrier to try and find something wrong.
    One more thing... Has it been verified that the A5 length receiver extension is the same length as the standard M&P10 tube? Just want to be sure, as mine wears an A5 tube. Buffer and spring are stock. The rifle locks back fine manually with room to spare, feels totally normal.

    Here she is... If only she would work!
    Last edited by masakari; 04-14-16 at 13:33.

  7. #17
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    So now your only issue is that the bolt won't lock back on empty with a couple kinds of ammo? Are you sure it was locking back with those ammo types before the mods? The M&P 10's gas port is sized to only function reliably on full house commercial .308 loads by design, and they generally won't lock back on mil spec/NATO ammo, which is a bit lower pressure.

    When I had mine, I drilled out the gas port a bit so it would run reliably on all ammo. Whatever you do, don't bungle the barrel, gas tube, or any other proprietary part of that rifle though. Believe me, you don't want to go down the road of trying to get replacement parts from S&W. BTDT.
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  8. #18
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    Quote Originally Posted by henschman View Post
    So now your only issue is that the bolt won't lock back on empty with a couple kinds of ammo? Are you sure it was locking back with those ammo types before the mods? The M&P 10's gas port is sized to only function reliably on full house commercial .308 loads by design, and they generally won't lock back on mil spec/NATO ammo, which is a bit lower pressure.

    When I had mine, I drilled out the gas port a bit so it would run reliably on all ammo. Whatever you do, don't bungle the barrel, gas tube, or any other proprietary part of that rifle though. Believe me, you don't want to go down the road of trying to get replacement parts from S&W. BTDT.
    It used to lock back, yes.
    And one of the loads tested was commercial Remington, the other commercial ZQI.
    I'm beginning to worry about disassembling and reassembling this rifle over and over again, especially as it relates to the gas block set screws and the friction fit of the Midwest industries rail. But I guess that's what I'll do, bring it to my Smith and have it drilled slightly.

  9. #19
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    You can also look at adjusting buffer weight.
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  10. #20
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    The problem is that I think the M&P10 comes stock with a carbine buffer. But maybe giving it more mass would help it reciprocate... Thoughts?
    Last edited by masakari; 04-14-16 at 16:52.

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