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

  1. #1
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    Installed MI MLOK rail and gas block on M&P10, accuracy and cycling issues

    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.
    Last edited by masakari; 03-29-16 at 21:01.

  2. #2
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    What have you checked so far? The first things that jump to mind are a loose barrel nut (accuracy issues) and/or a loose or misaligned gas block (cycling issues).
    Steve

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    Thanks.
    So, the barrel nut is definitely tight. It was actually somewhat difficult to get it tight enough to line up the rail and gas tube. But one notch looser was far too loose.
    The gas block feels tight, there is no movement then I try to twist it. The set screws were red loctited during installation.

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    Are you absolutely sure the gas hole of the new GB is aligned with the barrel's gas port?

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    Quote Originally Posted by Brahmzy View Post
    Are you absolutely sure the gas hole of the new GB is aligned with the barrel's gas port?
    I sure hope so. The gas block is straight up and down, I don't know how else I could check.

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    Did you measure the front/rear distance from the gas journal shoulder to the gas port and adjust the new gas block location accordingly? The gas block will typically sit a 1/16th off the shoulder. I've seen this alignment hose more guns than vertical alignment.

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    Quote Originally Posted by Brahmzy View Post
    Did you measure the front/rear distance from the gas journal shoulder to the gas port and adjust the new gas block location accordingly? The gas block will typically sit a 1/16th off the shoulder. I've seen this alignment hose more guns than vertical alignment.
    That could be the issue. I did not.
    Could that also effect accuracy?
    Getting that gas block back off is going to suck big time. And re dimpling the barrel... Oh man.
    Back to work!
    Last edited by masakari; 03-29-16 at 21:32.

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    Shouldn't affect accuracy. Did you mess with your optics while swapping parts out? You sure all your optics/mounts are tight? Weird simply swapping barrel nuts would affect accuracy, assuming everything's torqued properly. Gotta be something else. Make sure you've got plenty of 'free-float' distance around the gas block and the inside of the Handguard. There should be at least a couple millimeters.
    Once you get the gas block off, you'll see what I'm talking about with the forward/rearward gas port location alignment. It kind of depends on where the barrel maker drilled the hole. I've seen some drill it far forward and some drill it so far to the rear, towards the shoulder, that you HAVE to get the GB right up against the shoulder, with little to no room to spare. Even within the same high 'tier' barrel manus, this isn't constistant. Good news is, on a set screw gas block, the gas hole should line up exactly with the rear set screw hole, making that alignment easier to get right.

  9. #9
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    if your gas block is not aligned correctly then it will cause of the cycling issues. It can also effect accuracy since you are getting inconsistent gas. I had the exact same issue with a repair I made to a carbine (sub par gas block that was ill-fitting). Once I replaced the block, dimpled the barrel and reinstalled it everything was good to go.



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  10. #10
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    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?
    Last edited by masakari; 04-02-16 at 00:45.

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