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Thread: Measuring the barrel gas port size

  1. #31
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    Quote Originally Posted by Clint View Post
    If you want to measure small holes, I recommend a number drill bit set, like #60-#1.
    Number drills will get you in the ballpark, if that's all that's available.

    # dia., inches
    38 .1015
    39 .0995
    40 .0980
    41 .0960
    42 .0935
    43 .0890
    44 .0860
    45 .0820
    46 .0810
    47 .0785
    48 .0760
    49 .0730
    50 .0700
    51 .0670
    52 .0635
    53 .0595
    54 .0550
    55 .0520
    Oh no, not another lube thread! Read this first: Lubrication 101.

  2. #32
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    Quote Originally Posted by EzGoingKev View Post
    This guy lists different gas port sizes for the same length barrel but with different barrel diameters.

    Does that make sense?
    This is the list from the old Maryland AR-15 site, correct?

    A couple of things stand out:

    He's got the wrong gas port distance from the muzzle between the 14.5 and 16 inch barrels. He lists the same number, but that can't be since we all know the 16" has additional dwell.

    He does not account for mid-length setups, but this list is so old, it probably pre-dates mid-lengths coming on the market.

    I have no idea why gas port diameters would change between pencil barrels and standard ".750 under the gas block" barrels. I've always wanted to see an explanation on this, but when I asked years ago, I never got one.

  3. #33
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    Quote Originally Posted by EzGoingKev View Post
    This guy lists different gas port sizes for the same length barrel but with different barrel diameters.

    Does that make sense?
    Kind of, but it's not something I had considered. Thicker barrels have deeper gas port holes, so maybe he meant to even out the restriction by using a tighter hole on short holes and a wider hole on thick barrels.

    The difference in hole length from a .750 to .625 barrel is only .0625". If you think about it, blowing through a long skinny straw is harder than blowing through a short skinny straw.

    Would have to actually model this or set up some tests to be sure, though!
    Last edited by kartoffel; 09-01-11 at 09:32.
    Oh no, not another lube thread! Read this first: Lubrication 101.

  4. #34
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    Quote Originally Posted by jmart View Post
    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.

    Most people do not need to know exact details in regards to their GP size. Just a round about idea.

    Having measured enough barrels to know, people that own a RRA/BM/DPMS/Oly/S&W/Etc should accept the fact that their gun is over gassed and try to address it.


    C4

  5. #35
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    Quote Originally Posted by C4IGrant View Post
    people that own a RRA/BM/DPMS/Oly/S&W/Etc should accept the fact that their gun is over gassed and try to address it.

    C4
    just shoot nothing but underpowered wolf through them

  6. #36
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    Quote Originally Posted by militarymoron View Post
    just shoot nothing but underpowered wolf through them
    You know, I think that is reason why the cheaper guns are overgassed. Less problems with cheap ammo. Most sales are from the cheaper guns, so why not make them reliably shoot the cheaper ammo.

  7. #37
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    Quote Originally Posted by eagleeye View Post
    You know, I think that is reason why the cheaper guns are overgassed. Less problems with cheap ammo. Most sales are from the cheaper guns, so why not make them reliably shoot the cheaper ammo.
    This has been "known" in the industry forever. Meaning that RRA/BM/Oly/DPMS/etc assume that their customers are going to shoot the worst ammo they can find (read under pressured). So to reduce the number of CS calls they get about bolts failing to lock back, they over gas their guns.

    Now if we look at a company like LMT, they tell people that if they are shooting crap ammo, their guns aren't going to lock back (so deal with it). LMT makes guns for people that get into gun fights.

    I like LMT's CS advice the best.


    C4
    Last edited by C4IGrant; 09-02-11 at 09:45.

  8. #38
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    Quote Originally Posted by eagleeye View Post
    You know, I think that is reason why the cheaper guns are overgassed. Less problems with cheap ammo. Most sales are from the cheaper guns, so why not make them reliably shoot the cheaper ammo.
    Sounds smart and reasonable when you put it that way don't it?

  9. #39
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    I measure mine with the shank of a number size or fractional drill bit. I then verify the diameter of the shank with a micrometer.

    Not as exacting as some methods, but in my experience .001 either way does not make much of a difference.

  10. #40
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    Would not the actual diameter of the bore effect required gas port size as well? There is variance, hence the slugging of barrels to get a perfect cast bullet size. There is one of the newer guns I vaguely recall, though I think it was a piston setup, that was supposed to have a tapered bore or some such, I'd think that would throw pressure's off no? If so one would need to factor in the bore diameter to really compare gas port diameter overly much it seems to my layman self.

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