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Thread: 16" intermediate gas, gas port sizes. My SR15 is undergassed, lets talk ideal sizes.

  1. #21
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    Quote Originally Posted by Failure2Stop View Post
    Can you better define exactly what barrel you have?
    Pics will help.

    On the progress through the legacy, Mod 0, Mod 1, and Mod 2 guns there have been pretty significant changes to the system.
    It started life as Mod.0. The barrel has no external markings forward of the barrel nut. I don't think any marks are under here either. The only way it can be identified as KAC is by its gas system length and barrel extension.

    Quote Originally Posted by Clint View Post
    For future reference, can you measure the diameter of the bulb and report back?
    .174 on the new one. Behind the bulb is .165. The old one was .165 all the way with no visible evidence of a bulb ever being there.

    Quote Originally Posted by clandestine View Post
    I'm wondering if the aftermarket gas block is misaligned. I would have someone with a bore scope check it. It is how I check for port alignment.

    As far as the gas tube goes, brownells sells a gauge made by Mark Brown to measure the flared portion of the gas tube while it's still in the upper. It will find wear on the half that's not visible. It's a good gauge.
    Gas block is perfectly aligned. I verified this. The upper has been tried with a number of carriers. A JP lomas, LMT enhanced, the KAC that came with it, and now it has a KAC sand cutter carrier. One of these carriers must have had too small a gas key interior diameter and worn the tube. I say this because the bulb was evenly worn off all around the tube. No signs of pressure from one side.

    I am going to buy that tool based on this experience.

    Quote Originally Posted by Caduceus View Post
    Wasn't there something too, from KAC, about not running M193 spec ammo? I'll see if I can find it.
    I read similar, basically that it was tuned for heavier 62gr. green tip and there for does not always lock back with m193.


    So the gas tube is in, the old one had no bulb at all on it and had a slightly different bend in it. I'll post pictures in follow up. I won't be able to test fire for a week or two I don't think. But clearly there is a big difference in gas tubes. .175 for the new vs. old worn all the way to .164. I expect a big difference. If it is still a little weak for my taste I'll have the gas port opened to .08 or .082. I have a Noveske 18" intermediate barrel I'd like to measure the port on for comparison, its in a Vltor VIS so its kind of a PIA to get to. Pic to follow

    please click on pics for larger versions.
    Here are the two ends for comparison.
    2.jpg

    In these two pictures you can see where the bend in a different place, and how the old gas tube pointed down some.
    IMG_3729.jpg
    IMG_3728.jpg
    Last edited by TMS951; 01-15-19 at 15:57. Reason: pic

  2. #22
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    So a bit of an update here.

    I installed the gas tube and used my bolt repair kit. There was visible ware on the cam pin and extractor. The new extractor would not fit in my bolt. I contacted KAC and they said they have to special order one and its going to take a long time and they are going to send me another one. Currently waiting on that. In the mean time I used a knife sharpening stone and removed the high points on the extractor where I could see it was sticking.

    I managed to do a little very quick test firing. I only had time for 10 rounds. I fired 5 mags, with two rounds in each. # mags with federal fusion 62gr. .223 and two mags with xm193. All 5 mags locked back. The gun was well lubricated with ALG gojuice and very clean. It was 50 degrees out.

    I look forward to trying to run this rifle hard come spring and see what happens. At this point I will leave the gas port alone. I am going to try to did up some 55gr. 223 and see what it does on that. Theoretically by design it should not run well, I'm interested in how bad it will be.

  3. #23
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    You should run the weakest ammo, at the coldest temperature, holding the rifle loosely off your shoulder.

  4. #24
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    Quote Originally Posted by scottryan View Post
    I think it is time to part out this upper, sell off the questionable parts and buy new.
    This.^^

    And yes, KAC has directly said they do not recommend XM193, saying that the quality control is not good particularly with LC, leading to blown primers and the like (not because of bullet weight or pressure or anything). Personally I don't really buy that, never seen a problem with LC XM193 in any standard carbine.
    Last edited by SouthwestAviator; 02-07-19 at 10:23.

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