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Thread: Measuring the barrel gas port size

  1. #41
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    Quote Originally Posted by wolf_walker View Post
    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.
    OK, now I vote we're getting into minutia here !

  2. #42
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    Quote Originally Posted by jmart View Post
    OK, now I vote we're getting into minutia here !

    True enough. I foresee though, a post of "the gas port is tiny on this, but it still runs fine with an H-52 buffer and a spring out of a 85 chevy citation front strut in the buffer, why11!!111.

  3. #43
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    I made some adjustments to the stick and went to the range Monday.

    16" middy
    FA carrier
    XM193 and PPU75
    ETA: IMI 193
    A5 buffer

    Tested one size smaller insertable gas port.
    Dropped down to .073"

    This configuration ejected very consistently 3-3:30 with both loads.

    Verified lockback on 15-20 mags loaded with single rounds.

    I also tested two other buffer combos in the A5 tube.
    H2 + CAR spring + .750" spacer
    CAR buffer + CAR spring + .750" spacer

    Remarkably, the lighter buffers only moved ejection forward slightly, 3:00 and 2:30.

    I believe this level of gassing is near ideal, allowing a wide range of loads, buffers and springs to work well.

    These insertable gas ports work so well to properly fix overgassed barrels that I'm considering selling them.
    Last edited by Clint; 10-11-11 at 03:00.
    Black River Tactical
    BRT OPTIMUM HFCL Barrels - Hammer Forged Chrome Lined 11.5", 12.5", 14.5"
    BRT OPTIMUM Barrels - 16" MPR, 14.5" MPC, 12.5" MRC, 11.5" CQB, 9" PDW
    BRT EZTUNE Preset Gas Tubes - CAR and MID
    BRT Covert Comps 7.62, 5.56, 6X, 9mm
    BRT MarkBlue Gas Tubes - BRT EXT, EXC and PDW Lengths
    BRT MicroPin Gas Blocks - .750" & .625"
    BRT MicroTUNE Adjustable Gas Blocks
    BRT CustomTUNE Gas Ports

  4. #44
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    Quote Originally Posted by jmart View Post
    OK, now I vote we're getting into minutia here !
    well, this is a technical discussion forum after all. isn't that what it's here for?

    the fear of someone reading this and obsessing over minutia is their problem...
    never push a wrench...

  5. #45
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    It's not uncommon in testing to have ejection in the "correct" range, the bolt to lock to the rear and STILL have a cyclic rate I consider too low for reliability in adverse conditions.

  6. #46
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    Quote Originally Posted by Todd.K View Post
    It's not uncommon in testing to have ejection in the "correct" range, the bolt to lock to the rear and STILL have a cyclic rate I consider too low for reliability in adverse conditions.
    Agreed. Cyclic rate testing must be the more accurate method to determine gassing.

    Been meaning to ask you about that.

    Colt advertises the cyclic rate of the M4 and M16 at 700-950, while the M4 Commando is listed as 700-1000.

    Do you have your own target range(s) for factory new?

    Does these change based on total buffer weight and gas system length too?
    Last edited by Clint; 09-07-11 at 20:52.
    Black River Tactical
    BRT OPTIMUM HFCL Barrels - Hammer Forged Chrome Lined 11.5", 12.5", 14.5"
    BRT OPTIMUM Barrels - 16" MPR, 14.5" MPC, 12.5" MRC, 11.5" CQB, 9" PDW
    BRT EZTUNE Preset Gas Tubes - CAR and MID
    BRT Covert Comps 7.62, 5.56, 6X, 9mm
    BRT MarkBlue Gas Tubes - BRT EXT, EXC and PDW Lengths
    BRT MicroPin Gas Blocks - .750" & .625"
    BRT MicroTUNE Adjustable Gas Blocks
    BRT CustomTUNE Gas Ports

  7. #47
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    Seems to me if you are mechanically inclined you already have a fractional and numbered drill bit set. That should get you pretty close. Then just order the gauges between the one that fits and the one that doesn't. Shouldn't be much more than $10. Does it matter a couple of thousandths? Then what are you going to do with the information? Do you really know what size the port should be? I don't really recall a topic/thread that addressed the issue.
    Last edited by hals1; 09-08-11 at 01:06.

  8. #48
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    Quote Originally Posted by Clint View Post
    Tested one size smaller insertable gas port.
    Dropped down to .073"

    These insertable gas ports work so well to properly fix overgassed barrels that I'm considering selling them.
    Photos or it doesn't exist...

  9. #49
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    Quote Originally Posted by MK18Pilot View Post
    Photos or it doesn't exist...
    +1 on that.

  10. #50
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    You should be able to get + or - .001 with a good caliper. A good digital caliper is also a lot more useful for other things than a range of gauge pins.

    Nothing wrong with a set of gauge pins but if you are on a budget the calipers will be the better buy based on utility but one has to wonder what most will do if their port is “Oversize”. Live with it is my guess but I am interested in any data other are willing to share.

    Plus or minus .001 will have absolutely no effect on function so this level of measurement resolution is more than sufficient. I just measured 2 carbine length gas ports and the range was from .068 to .078 so you can see the range that might be encountered. I suspect some of the grossly overgassed would even exceed the 0.080 in the 7 inch gas systems.

    Here is my data from the barrels laying in the shop:

    1990s vintage Bushmaster 11.5 barrel with 7 inch gas and 0.750 seat was 0.078 and the gun ran fine from what I remember and was not particularly hard on brass.

    YHM 16 inch barrel with 7 inch gas and 0.750 seat with 0.068 port diameter and ran fine but I used an adjustable block to throttle is back a bit. Wide open the .068 port was reliable with xm193 and Wolf but hard on brass in temps from 30 degs to 100 degs at 5000 ft altitude and run wide open.

    Just guessing here as I never shot that barrel at really cold temps(-20 F or lower) but .223 pressure loads would most likely work well down to some pretty low temps with the 0.068 port diameter and it might take a serious cold snap to stop xm-193.

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