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Thread: Carbine vs. Middy: Real world use

  1. #41
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    Quote Originally Posted by MistWolf View Post
    Because the smaller port size restricts the volume (CFM, or Cubic Feet per Minute) of the gas passing through. Just as a small compressor can supply enough pressure to inflate a tire, with a low CFM it will take awhile. Without enough volume, pressure cannot be built up inside the bolt carrier quickly enough to provide reliable functioning
    This^^ a plain hole in the barrel is not a psi regulator it just a restrictor .
    So many fail to realize this.

  2. #42
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    I tend to believe that gas port erosion happens and moving the port farther forward is a good thing.

  3. #43
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    Quote Originally Posted by condition1 View Post
    Interesting. I was under the assumption that being overgassed would not be a good thing and would result in reliability issues.
    Yes, which is why people will throw donuts around their extractor springs and other mods.

    In the end buy a good rifle that has the features you like, shoot it, if it works great, if not fix it or get another.

    We use 6920 carbines at work and have no problems.
    Last edited by cacop; 03-27-11 at 00:52.

  4. #44
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    Quote Originally Posted by RyanB View Post
    I tend to believe that gas port erosion happens and moving the port farther forward is a good thing.
    Your barrel erodes everytime you expose it to air. :O Erosion happens, it's just not as big of a deal as it get's made out to be most of the time. Mag dumps on a suppressed SBR, yes, definite concern, semi auto aimed fire, not so much.

  5. #45
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    Somewhat of a semantic argument, it actually is a regulator. Any air regulator is just a needle that opens and closes into a tapered orifice changing the size of the opening. A gas port just happens to be a fixed size. A smaller gas port will have a higher pressure right at the gas port (gasses compress so pascal's principle doesn't apply.) but it will restrict volumetric flow, thus requiring more time before peak pressure is reached. Opening up the gas port results in less pressure, again, right at the gas port, but peak pressure will be reached faster.

    Any carbine requires a specific amount of internal pressure to begin unlocking and as long as the bullet is still in the barrel and the bolt is locked, pressure continues to build. In the case of a carbine, pressure is a function of volume and temperature. Volume is a function of gas port size. The gas port is sized to allow enough volume of gas into the bcg to generate necessary unlocking pressure at the proper time.

    Quote Originally Posted by Thomas M-4 View Post
    This^^ a plain hole in the barrel is not a psi regulator it just a restrictor .
    So many fail to realize this.
    Last edited by Eric D.; 03-27-11 at 01:12.
    B.A.S. Mechanical Engineering Technology

  6. #46
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    The gas port isn't a regulator, it's a restrictor. A regulator regulates a higher pressure from one side to a lower pressure on the other. When the system is fully charged, the low pressure side will not exceed the pressure set by the regulator although the pressure on the high side is higher.

    A restrictor doesn't reduce the PSI, it simply controls how fast the gas will pass through. When a system with a restrictor is fully charged, the pressure will be equal on both sides
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
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  7. #47
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    Zomg it's an orfice plate. >.>

  8. #48
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    Quote Originally Posted by Eric D. View Post
    Somewhat of a semantic argument, it actually is a regulator. Any air regulator is just a needle that opens and closes into a tapered orifice changing the size of the opening. A gas port just happens to be a fixed size. A smaller gas port will have a higher pressure right at the gas port (gasses compress so pascal's principle doesn't apply.) but it will restrict volumetric flow, thus requiring more time before peak pressure is reached. Opening up the gas port results in less pressure, again, right at the gas port, but peak pressure will be reached faster.No shit that's how you get an over gassed carbine.It does not regulate it times

    Any carbine requires a specific amount of internal pressure to begin unlocking and as long as the bullet is still in the barrel and the bolt is locked, pressure continues to build. In the case of a carbine, pressure is a function of volume and temperature. Volume is a function of gas port size. The gas port is sized to allow enough volume of gas into the bcg to generate necessary unlocking pressure at the proper time.
    Right you just proved my point it is not a regulator its a restrictor

  9. #49
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    Quote Originally Posted by ZRH View Post
    Zomg it's an orfice plate. >.>
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
    I am American

  10. #50
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    I am not expert on gas port sizes but this is where my comment about making it smaller comes from.

    Colt uses a .061-062 sized gas port on their 6920 that comes standard with a 16" barrel.
    Colt uses a .070 gas port on their MK18 with a 10.5" barrel.
    These two things lead me to believe that a shorter barreled rifle needs a larger gas port than a longer barrel to function.

    There are a few posts on this board with guys running SBR's with gas ports in the high .050's that function reliably. And that is without a suppressor installed.

    This makes me think that .061 might not be the smallest port size a 14.5-16" barrel needs to operate.

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