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Thread: 10.3" malfunctions, my fault or build problems?

  1. #1
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    10.3" malfunctions, my fault or build problems?

    Today I finally took out my 10.3" build and I experienced some problems while shooting it. The build is as follows.

    10.3 DD barrel with .078" gas port
    BCM mod 0 gunfighter comp
    Micro MOA Govnah gas block with .070"/.078" ports
    BCM KMR 9
    BCM BCG
    BCM upper
    H3 buffer
    USGI buffer spring

    I had no issues what so ever using the .078" port, but I had a few problems when I switched to the .070" port.

    with federal xm193 I had one bolt over base malfunction and one true double feed, trying to feed two live rounds.

    with pmc 223 and tula 223 I had at least 5 short strokes and failure to lock back on the last round.

    I was a bit shocked I had these problems especially with the xm193 and all the while I thought I had an H2 buffer in there as opposed to the H3. Do you think that extra weight would be enough to cause the problems I had with the xm193? When I arrived home I took off the KMR to verify that the ports were in correct order and they were, I even double checked the port size. Now that I think about it, it might be possible that the bolt over base malfunction was with the large gas port but I am not 100% sure, maybe, but I doubt it. This was also with at least 7 different mags
    Last edited by black22rifle; 04-30-15 at 17:46.
    If there is no enemy within the enemy outside can do us no harm

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    I am assuming that you're using a standard length carbine buffer tube. I would try the H2 again, or even a H buffer to see if that cycles better. I run an H in the same setup and it works flawlessly now. Only if I run crappy Tula or something (not ever) I will get the short stroking problems, and no lock back on last round.
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    I was mistakenly using an H3 when I thought I had an H2 in, but I didn't realize it until I got home.
    If there is no enemy within the enemy outside can do us no harm

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    Quote Originally Posted by sf3pox1 View Post
    I am assuming that you're using a standard length carbine buffer tube. I would try the H2 again, or even a H buffer to see if that cycles better. I run an H in the same setup and it works flawlessly now. Only if I run crappy Tula or something (not ever) I will get the short stroking problems, and no lock back on last round.
    IIRC, Black River Tactical says his gas port inserts act as though they are slightly larger than size (.073 insert acts like a .070 gas port). I wonder if using a MircoMOA with a .070 gas port has a similar effect. Also, I would try an H2 buffer. If the symptoms continue, then try an H.

  5. #5
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    You have an interesting combination of circumstances going on here. I can't help but wonder if they're all related of if you could have different problems causing different malfunctions. Replies in red.

    Quote Originally Posted by black22rifle View Post
    Today I finally took out my 10.3" build and I experienced some problems while shooting it. The build is as follows.

    10.3 DD barrel with .078" gas port
    DD MK18 barrels are known for being overgassed. 0.078" gas port is on the large side.
    BCM mod 0 gunfighter comp
    Micro MOA Govnah gas block with .070"/.078" ports
    See above regarding the larger port.
    BCM KMR 9
    BCM BCG
    BCM upper
    H3 buffer
    This is on the heavy side.
    USGI buffer spring

    I had no issues what so ever using the .078" port, but I had a few problems when I switched to the .070" port.
    With this gas port size, it's apparently overgassed enough to run weak ammo even with the heavy buffer.

    with federal xm193 I had one bolt over base malfunction and one true double feed, trying to feed two live rounds.
    True double feeds are almost always caused by a bad magazine. Did you have the double feed and bolt over base malfunction with the same mag?

    with pmc 223 and tula 223 I had at least 5 short strokes and failure to lock back on the last round.
    This is some of the weakest ammo out there, which is likely the reason for these issues. The Crane spec for a MK18 is an H2 buffer and 0.071" gas port meant to be utilized with true 5.56 ammo. You're running a slightly smaller gas port, a heavier buffer, and weaker ammo.

    I was a bit shocked I had these problems especially with the xm193 and all the while I thought I had an H2 buffer in there as opposed to the H3. Do you think that extra weight would be enough to cause the problems I had with the xm193? When I arrived home I took off the KMR to verify that the ports were in correct order and they were, I even double checked the port size. Now that I think about it, it might be possible that the bolt over base malfunction was with the large gas port but I am not 100% sure, maybe, but I doubt it. This was also with at least 7 different mags
    Could you isolate different types of malfunctions to different mags? What kind of mags are they?
    Last edited by SteveL; 04-30-15 at 20:42.
    Steve

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    No the double feed and bolt over base were with separate mags, actually all malfunctions were with separate mags. The short strokes were with either USGI D&H or CPD mags, but the double feed and bolt over base were with PMAGs. Just out of curiosity how does crane get a .071" port if there is no .070" drill bit size? The next size up from .070" is .073".
    If there is no enemy within the enemy outside can do us no harm

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    Try the single round lock back test to narrow it down the actual symptoms.

    The gas port probably needs to be bumped up to .073 and the buffer down to H2.
    Last edited by Clint; 05-01-15 at 14:50.
    Black River Tactical
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  8. #8
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    Quote Originally Posted by black22rifle View Post
    No the double feed and bolt over base were with separate mags, actually all malfunctions were with separate mags. The short strokes were with either USGI D&H or CPD mags, but the double feed and bolt over base were with PMAGs. Just out of curiosity how does crane get a .071" port if there is no .070" drill bit size? The next size up from .070" is .073".
    It's 1.8 mm and you can get bits in that size.
    Steve

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    Page 4 of the Crane presentation says .070 not .071. http://www.dtic.mil/ndia/2003smallarms/john.ppt

    The commercial DD 10.3's I've seen have been .081~.082 while the 'milspec' ones have been .070".
    President of MicroMOA and a corporate member of Artisan Arms

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    Quote Originally Posted by MicroMOA View Post
    Page 4 of the Crane presentation says .070 not .071. http://www.dtic.mil/ndia/2003smallarms/john.ppt

    The commercial DD 10.3's I've seen have been .081~.082 while the 'milspec' ones have been .070".
    I'm sorry if my number was off. However, it doesn't change the problem the OP is having. The combination of the small gas port with heavy buffer and weak ammo is likely causing the short stroking and failure to lock back after the last round.
    Last edited by SteveL; 05-19-15 at 18:11.
    Steve

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