HKMR556 10.4" Conversion cycling issues
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Thread: HKMR556 10.4" Conversion cycling issues

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    Default HKMR556 10.4" Conversion cycling issues

    My son acquired a new MR556 for xmas 19' while on leave. Shortly after he left it went straight to Marvin Pitts of Nefarious Arms, to be chopped to 10.4" and re-profiled. Just picked it up on the 27th of June and we function tested it before he went back. The rifle fires and extracts at about 3 o'clock but does not pick up the next round. Admittedly it feels stiff when running the action. I did some testing, the factory buffer is 5.01 ounces, so somewhere between an H2 and H3. The factory HK buffer spring is shorter than a mil-spec spring but noticeably stiffer. Ammunition is Federal XM855 62 Grain Green tips and should be hot enough.

    As a test I also ran a Larue Spring, that I found out from springco, is their spring, that is stiffer then a mil-spec spring but in my gross estimation weaker then the HK spring. I ran this spring with an H2 Buffer, HK Buffer and a stander mil-spec buffer, all with the same results as above.

    Second test was with the standard LMT Mil-spec spring, an H2 buffer, HK buffer and a standard mil-spec buffer. The gun functioned the same as above, except for with the standard mil-spec buffer, in which it function flawlessly and ejected at 4 o'clock as it should.

    From my readings thus far, a really real world HK416 is supposed to be over gassed for reliability. I'm not sure what the MR556 gas port spec is but I'd assume the true 416 is larger...? I've tried getting a hold of Marvin to see if he'd opened the gas port up or if he thought it needed it, but I've left two messages. I could order the mil-spec spring and buffer but I'm not a fan of the firearm needing such weak tolerances to run. I'd rather it be over-gassed and deal with the issue accordingly. Additionally, isn't the HK buffer proprietary?

    Any insight would be greatly appreciated.
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    @HKCS S perhaps? I know it's not your problem due to the modification, any insight or recommendations?

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    Just my 2 bits here . If the gun function flawlessly with the mil spec buffer and LMT spring, then you should stick with that combo . My opinion is , if the gas port is small and thus the gun is under gassed, the buffer and spring have to be lighter for cycling the bolt back for proper timing !!
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    @sabre675

    Did Marvin open the barrel port ( to 1.6mm ) and weld up and redrill the gasblock vent?
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    Quote Originally Posted by kbinmt View Post
    @sabre675

    Did Marvin open the barrel port ( to 1.6mm ) and weld up and redrill the gasblock vent?
    Still trying to make contact. I'll ask him when I do. Thanks for the response.

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    Ditto, correct gas port size and GB is the most important part of an MR556 conversion. Let us know what you find out.
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    Spoke to Marvin today. My dumbed down version of the conversation is that he does not want to weld and re-drill the gas block as he as had mixed results in doing so....

    So here are the options as I understand them.

    1. Simply use the mil-spec spring and standard buffer as I've listed above as it seems to function properly. (suppressed dunno).. *Option #1 is what Marvin Pitts advised me he would do...
    2. Send it back to him and he would weld the vented portion of the gas block... Simply pay return shipping.
    3. Send a blank HK Gas Block in and he would drill it... WTF do you find one of these
    4. Order the HK 416 10.4 Gas Block off of HKparts.net for $250 that is appropriate for the build..However I don't know if this would work as I don't know if the gas port in the barrel is appropriate to the block...

    Is this tracking with anyone? Thoughts?

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    MR556 barrels need a welded up vent hole gas block in order for it to function properly with its intended proprietary spring/buffer combo.

    I don't have any experience with an original non-vented 416 10" gasblock, but I would assume it would have the same affect.

    I personally would not mess with the gas port size on the MR556 barrel especially when cut down, unless you are going to run a factory unicorn 416 10" vented gas block in which then you enlarge it to 1.6mm. I also would not run anything other than the stock HK spring/buffer as that is what makes it very special and unique to the rifle.

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    I had the port on my MR556 cut down barrel opened to 1.6mm. It runs great in both unsuppressed and suppressed modes with my AGB. I have a 10” vented block that I plan to use on the barrel from Nefarious Arms. I haven't tried a non-vented 10” gas block.

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    Quote Originally Posted by LEEF View Post
    MR556 barrels need a welded up vent hole gas block in order for it to function properly with its intended proprietary spring/buffer combo.

    I don't have any experience with an original non-vented 416 10" gasblock, but I would assume it would have the same affect.

    I personally would not mess with the gas port size on the MR556 barrel especially when cut down, unless you are going to run a factory unicorn 416 10" vented gas block in which then you enlarge it to 1.6mm. I also would not run anything other than the stock HK spring/buffer as that is what makes it very special and unique to the rifle.
    You can use a 14.5”/16” vented gas block on a 10.4” barrel. You just need to have the barrel gas port opened up accordingly. You have to find someone who knows the correct diameter.
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