MR556 10.4" Not Cycling
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    Default MR556 10.4" Not Cycling

    Just purchased a 10.4" MR556 upper from another member and I threw it on an ar pistol lower with the HK buffer and buffer spring. All the other parts are MR556 LPK as well. I am having an issue where everytime I fire a round, the round ejects but the rifle does not cycle the next round. I am however able to pull the trigger again for it to go click.

    I am also using PMC rounds. Is it the round? Would a heavier buffer work?

    The PMC rounds was the 59 gr and I also used the Herters 62gr and on the Herters, it wouldn't eject the round after firing.

    The gas block is vented as well.
    Last edited by zushikikato; 05-26-2018 at 10:05 PM.

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    Will it lock open in the last round? What is the vent port size?
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    Based off the Herter's, it sounds more undergassed than over-gassed. You can eject but fail to feed the next on an under-gassed gun...I have seen this several times with Mk12/SPR's running non-Mk262 in cold temps. I don't believe you're on the other end akin to suppressed guns out-running the mag springs (that would require a heavier buffer).

    I'm going to take a wild guess that they didn't plug the bleed-off valve in the gas block if they used the stock MR556 gas block...
    Also, depending on which PMC you're using (Bronze?...XTAC?), I've seen PMC put up some pretty feeble MV numbers and it seems pretty weak.

    There is also some consideration in how you run it. Without getting into all of the stupid ATF "shouldered" mumbo jumbo, understand this. The AR/416 is meant to be shouldered/driven. IF you actually shoot an AR as you would a pistol, it exposes undergassed/underpowered guns. In fact this is often a test we use when tuning "tamed" guns to ensure that we are not walking into areas of unreliability. If your gun can be shot pistol/isosceles style, that's a good indication that your gun has reasonably sufficient cycling.

    Finally, what mags are you using and did you try other magazines?

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    Quote Originally Posted by CIMG View Post
    Will it lock open in the last round? What is the vent port size?
    It will not lock open in the last round. Not sure of the vent port size but it looks wider than the other mr556 gas block i have

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    Quote Originally Posted by dms16 View Post
    Based off the Herter's, it sounds more undergassed than over-gassed. You can eject but fail to feed the next on an under-gassed gun...I have seen this several times with Mk12/SPR's running non-Mk262 in cold temps. I don't believe you're on the other end akin to suppressed guns out-running the mag springs (that would require a heavier buffer).

    I'm going to take a wild guess that they didn't plug the bleed-off valve in the gas block if they used the stock MR556 gas block...
    Also, depending on which PMC you're using (Bronze?...XTAC?), I've seen PMC put up some pretty feeble MV numbers and it seems pretty weak.

    There is also some consideration in how you run it. Without getting into all of the stupid ATF "shouldered" mumbo jumbo, understand this. The AR/416 is meant to be shouldered/driven. IF you actually shoot an AR as you would a pistol, it exposes undergassed/underpowered guns. In fact this is often a test we use when tuning "tamed" guns to ensure that we are not walking into areas of unreliability. If your gun can be shot pistol/isosceles style, that's a good indication that your gun has reasonably sufficient cycling.

    Finally, what mags are you using and did you try other magazines?
    The valve is not plugged off on the gas block (Its the giant hole in the gas block right?)

    I do want to eventually run this with a suppressor and be able to run it with and without one.

    I am shooting it shouldered.

    I tried using the HK polymer mr556 magazines as well as pmags m3 windowed.

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    Try some legit spec 5.56 ammo like real m855 or a black hills load. Maybe black hills 77gr (red box).
    If it won’t eat that then you will need to make temporary adjustments until you get your can.

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    If he has a vented gas block then one way to fix it is to have the barrel gas port tuned to the vented block (meaning having it slightly bored out to accommodate the vented gas block. I’ll bet it’s a 14.5”/16” vented block.

    I’d ask the seller the origin of the gas block, chances are it’s the original ME556 16” barrel gas block.

    If so, then it will never cycle correctly in that configuration without a suppressor.

    There are some really good armorers on here who can do the work to make it function correctly (@InshallahTech).
    Last edited by RegularGuy461969; 05-28-2018 at 11:38 AM.
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    Barrel cut, standard 14.5/16" gas block, yup, yer gonna have a bad time.

    Either open the barrel port or get a non vent gas block.

    If you plan on suppressing, get a proper 10.4 vented gas block AND open the gas port to spec.

    I'm not sure why someone on this board would sell an item knowing that it wouldn't work properly without fair warning... :-/
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    Couldn't you try a lighter buffer/spring combo?

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    @InshallahTech

    Isn’t it possible to tune the cut down 10.4” barrel to work with a 14.5”/16” gas block (suppressed and unsuppressed)?

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