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Thread: Measuring the barrel gas port size

  1. #21
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    Quote Originally Posted by jmart View Post
    This level of minutia is probably more relevant to how the carbine functions than a lot of other things that folks obsess over on this board.
    Correct, but most people would be better served buying a quality barrel and using the buffer and spring the barrel manufacturer recommends (or just buy a complete rifle).

    Sure you can "tune" a carbine by how the recoil "feels" or where the brass ejects to but cyclic rate testing is far more precise, and not an option to most.

  2. #22
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    Quote Originally Posted by rob_s View Post
    Feeler gauges? I thought feeler gauges were flat metal used for measuring gaps?

    There are flat ones and round ones. They also go by several other names.


    C4
    Last edited by C4IGrant; 08-31-11 at 13:41.

  3. #23
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    Quote Originally Posted by rob_s View Post
    I would also like to hear from people that have measured, or think they have measured, some ports and what kind of size range you've encountered. I've seen posts mentioning down to 0.060 and up to 0.090, which would be a total of 31 pins at .001 increments. I'd hate to buy $60 worth of pins only to find out I need one two sizes smaller, or two sizes larger.
    The mil-spec guns are going to be right around the same measurments (on the carbine side). So I would get something from say .062-.064.

    For the non-milspec guns (BM, RRA, DPMA, S&W, Oly, etc), I would get ones starting at about .069-.078.


    C4

  4. #24
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    On a related note, I think it would be very interesting to have a chart of the gas port sizes for all of the various top-tier AR manufacturers, like BCM, DD, Colt, Noveske, etc. Ideally, the chart would include gas port sizes for all of their gas system/barrel length combos.

    It would be even better if we could get each of the manufacturers to describe WHY they chose that gas port size.

    Chris
    Last edited by Chris17404; 08-31-11 at 14:53.

  5. #25
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    The ones Grant is talking about are called wire feeler gauges usually used to measure spark plug/ignition gaps. I haven't found a set that goes in small increments or that big of a gap.
    Last edited by johnson; 08-31-11 at 15:19.

  6. #26
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    Quote Originally Posted by rob_s View Post
    ...we should determine how people are making their measurements and assumptions.
    I have Daniel Defense lightweight barrel with a mid-length gas system and the port size is .072".

    I determined the size by using pin gauges -



    And then confirmed the pin gauge sizing with a micrometer -



    The funny thing is the package it came in states it has a carbine length gas system but it is a mid-length.


    Quote Originally Posted by rob_s View Post
    While I am generally opposed to the amount of attention paid to this level of minutiae....
    Rob_S,

    This is not an attack, I am not "butthurt", nor do I have a bone to pic but when reading that statement and thinking about some of the things you have been concerned with I rolled my eyes.

  7. #27
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    Quote Originally Posted by C4IGrant View Post
    The mil-spec guns are going to be right around the same measurments (on the carbine side). So I would get something from say .062-.064.

    For the non-milspec guns (BM, RRA, DPMA, S&W, Oly, etc), I would get ones starting at about .069-.078.


    C4


    If you're not in a hury, I'd remove the gas block/FSB and then use whatever you have on hand to "get close". Use a caliper, use a drill bit, whatever. From that dimension, then I'd measure and order gauges covering .002/4" under-to-0.002/4" over in .002" increments. That way you don't have a bunch of needless gauges on hand.

    I'm no gunsmith so I'm not too keen on ordering a bunch of gauges just to have them on hand to cover the gamut of whatever might come my way. Alternatively (for carbines, not sure what I'd order for mid-lengths, so the initial measuring would be necessary) I'd just order .060" - .074" or thereabouts in .002" steps and that should cover what I'd be interested in learning. Anything gauging above my max pin gauge would let me know I'm way overgassed and I ought to be looking at heavier buffers and stiffer springs.

    Again, I'm no gunsmith, but I don't know how important it is to know your exact gas port dimension, but it would seem to be important to know if you are in the range of correct port size vs over-sized. Not really sure where that demarcation would fall, but I'm pretty certain that if I knew I were .010" over spec, then I'm in the overgassed range and need to take some mitigating action. Maybe .005" over lands you in this range, maybe .007" is the number, that's another discussion. If enough people reported their:
    • port dimension

    • gas system length

    • buffer weight

    • carrier type (FA vs SA)

    • buffer spring type (stock vs CS extra power)

    • type of ammo they shoot

    • and how well the gun runs with all that considered together


    then we'd develop a database that in time would lead us in the right direction. Maybe that's too much to ask.

    This would probably be more useful for the non-Colt/BCM owners out there because you can probably assume that the Colts/BCMs are running in-spec, or very close to in-spec, ports. It would also validate whether or not the BM/RRA/Armalite/et al owners do indeed have over-gassed weapons or if these barrels too are in-spec.
    Last edited by jmart; 08-31-11 at 20:07.

  8. #28
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    If you want to measure small holes, I recommend a number drill bit set, like #60-#1.

    It is not exactly like a pin gauge set with sizes every thou, but works because holes are made by drill bits, in the sizes of standard drill bits, which follow the standard wire sizes.

    Use the shank end to check fit and double check the diameter with a caliper or better yet, micrometer.

    But honestly, if you can measure it within a thou, you are good enough for this application.


    Also, I don't think this thread will make many Mfgs happy, as they consider gas port sizes proprietary and probably spent a great deal of time and ammo arriving at this number.

    For example, how big is the gas port on a 14.5" BCM middy?

    You won't find the answer here.


    Caliber plays a big role as well, and should be noted if not 5.56.

    The pressure curves of 6.8x43 or 7.62x35 are much faster than 5.56x45 and requre larger ports or closer gas port locations.
    Last edited by Clint; 08-31-11 at 23:03.
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  9. #29
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    Based off of what I know some of those numbers are seriously jacked.

    Quote Originally Posted by rob_s View Post
    I have no way of verifying/vouching but this post has some interesting information.

    http://arizonashooting.com/v3/viewtopic.php?f=8&t=12922



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  10. #30
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    http://arizonashooting.com/v3/viewtopic.php?f=8&t=12922

    Barrel Length (in) Barrel Diameter (in) Distance from Muzzle (in) Min Port Size (in) Max Port Size (in)
    11.5 .625 3.850 .081 .089
    11.5 .750 3.850 .086 .094
    14.5 .625 8.375 .063 .078
    14.5 .750 8.375 .070 .086
    16 .625 8.375 .063 .078
    16 .750 8.375 .070 .086
    20 .625 6.875 .086 .093
    20 .750 6.875 .093 .096
    24 .825 N/A .089 .089
    This guy lists different gas port sizes for the same length barrel but with different barrel diameters.

    Does that make sense?

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