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  1. #11
    MGO Member Moleman-'s Avatar
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    I made a few 16" middy barrels this past spring. With a standard 3 0z buffer they would cycle but not lock back the bolt with a .073" (#49) drill bit. They seemed to function fine with a #47 (.078") gas port but the range goes up to .085" for a 16" barrel, and I'd expect a shorter barrel would need at least that if not a little bigger. Pressure/volume/time needed to cycle. Make one leg shorter the other two have to compensate for it if they can.

  2. #12
    MGO Member Moleman-'s Avatar
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    I've run across a few gas blocks where the gas tube hole was partially blocked. Take a #32 .116" drill bit and open up the tube if needed. Having an egg shaped gas tube port hole is better than having the port partially blocked.

  3. #13
    I am a Forum User
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    I went back to the range today. Brought a couple other AR's with me. Did a lot of part swapping. The results are extremely ODD!!! My head hurts. Most comparative testing was done with a complete BCM 16" midlength.

    1. Complete BCM upper on my trouble gun's lower= success with lower pressure ammo
    2. Different BCG in my trouble upper= failure BUT..... if I put trouble upper BCG into good tested upper IT STILL FAILS........WTF?!?!
    3. Trouble upper fails with 3 different complete lowers of varying buffer systems concluding the lower is not the issue in any way really.

    So it seems that I obviously have a gassing issue(s). It seems I ALSO have a BCG issue as well?!?!
    It still cycles XM193 and M855 reliably and nice, but nothing else. Thats a little too finicky for my taste.
    I will be taking upper apart next. Thoughts on the BCG too!??!?! Looks staked fine.

  4. #14
    MGO Member Moleman-'s Avatar
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    There's a lot of places within the bolt carrier and on the bolt that if they are not within a certain range will cause the BCG to leak too much gas around the bolt tail or at the rings. Here's a video that explains that https://www.youtube.com/watch?v=CrJl3LThLFw

    Generally if I have a BCG that doesn't work well on a given upper (I do lots of straight walled wildcats where gas is an issue) I'll swap it for one that will work and try to use the less than ideal BCG on a 556 20" rifle with standard gas which seem to be more forgiving.

  5. #15
    MGO Member
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    Check the gas rings ...... on the bolt ..... indexing and wear . DO the test to see if they need replacing..

  6. #16
    I am a Forum User
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    bet you have gas key leakage on the BCG. had that issue with one of mine, and after the maker swapped it out, good to go.

  7. #17
    Marketplace Moderator
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    If you are still instumped, call American Arms Engineering. He is a milspec armorer. He worked as a contractor for Blackwater as their armorer. He was also the contract armorer for the 25th Infantry Division. He knows these rifles well. He helped me out in the past. He is located near Armada. His name is Ron. 15869446577. If worst comes to worst, he can get rid of your headaches.
    Good Safe Shooting, MOOSE

  8. #18
    In Memoriam tenthumbs's Avatar
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    I meant to post this in your thread. Interesting and worth watching (I think) but not sure it addresses your particular issue.

    https://www.migunowners.org/forum/sh...18#post3331518

  9. #19
    I am a Forum User
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    Thank you everyone for the helpful posts. I definitely had a strange issue with bcg and sent it in to BCM but that shouldn't be my root cause because it happens with a different bcg in it as well....... I'm baffled. I took everything apart and the gasblock screw hole was definitely directly over the dimple so I have to assume it is aligned. I cannot take off gas block because muzzle is pinned...... I'm thinking of swapping gas tube just in case and because I literally don't know what else to try.

  10. #20
    MGO Member
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    If you bought with a pinned muzzle device I'd be sending it back to the manufacturer for replacement or fixing. Something is definitely not right with the gas on this thing.

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