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  1. #1
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    Gas port wear timeline question

    My MK18 has recently started to have malfunctions at least once per range session that looks just like an over-gassed system. Empty brass will will come back into the ejection port, primer first, and impinge the round going into the chamber and the bolt. Will typically happen with a specific mag when suppressed, so guessing the spring is about done on that mag (M2 with who knows how many thousands of rounds), but it has also happened with a brand new (<500 round) D60 when unsuppressed.

    What has me puzzled is that the barrel only has 3100 rounds (1800 of which were suppressed). Action spring also only has about 2700 rounds on it. That round count seems a little low for the gas port to be having excess gas, so was wondering if others would agree.

    Barrel is a Colt 6920 cut down by ADCO with their gas port size adjusted. Buffer is a H2, per the Crane spec. The one other variable is a WMD BCG (put in back when I used to think NiB was "the thing").

    Obviously I can try a different bolt (to test the extractor) and put in a H3 buffer, but the intent of my question here was more about port wear, because again, that seems low for this to be popping up now. Thoughts?

  2. #2
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    Doesn't sound overgassed to me, unless the brass is hitting the front of the ejection port. The recoil would be noticeably sharper, too.

    Rather, it sounds like your ejector (not extractor) needs to be cleaned or time to change the spring.

    Throw a different bolt in and see what happens.
    Will - Owner of Arisaka LLC - http://www.arisakadefense.com

  3. #3
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    "Action spring also only has about 2700 rounds on it." If you mean buffer spring and its just a standard spring they only last around 3K rounds.

    Give the rifle a spring tune up:

    New: Buffer spring, ejector spring/ejector, extractor spring/buffer and O-ring, and new gas rings. New extractor wont hurt either. About $25...

    I run my rifles pretty hard and do this every 3000 rounds. I would rather be a little early than push consumables past their breaking point.
    Doesn't sound like port wear. Its more like a timing issue. All the springs work in conjunction with one another, when one starts to wear down it affects the timing of the rifle. Maybe not real noticeable at first but more so sooner than later. That's why I change all those parts out at one time. Even extractors have a life of 3- 5000 rounds.
    Last edited by Stone; 19 October 2018 at 16:38.
    The best way to survive a violent encounter is to be the one inflicting the most violence.

  4. #4
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    Quote Originally Posted by Slippers View Post
    Doesn't sound overgassed to me, unless the brass is hitting the front of the ejection port. The recoil would be noticeably sharper, too.

    Rather, it sounds like your ejector (not extractor) needs to be cleaned or time to change the spring.

    Throw a different bolt in and see what happens.
    Hmm, it seems to be doing the same thing that two other rifles were doing that were known over-gassers...a 308 that you helped me diagnose, and my 416. Although admittedly, a big part of the 416 issue seemed to be mag spring related, as well. Going to M3 mags fixed everything.

    The brass is getting thrown forward to about 1 o'clock. I'm not locked into the whole "brass ejection chart" thing, but just putting it out there. That said, I seek some education. What is it about the ejector/spring that would cause an issue like this? I'm not arguing your opinion, but I've never totally understood how ejector health fit into the timing of the system.

    Quote Originally Posted by Stone
    If you mean buffer spring and its just a standard spring they only last around 3K rounds.

    Give the rifle a spring tune up:

    New: Buffer spring, ejector spring/ejector, extractor spring/buffer and O-ring, and new gas rings. New extractor wont hurt either. About $25...
    Fair enough. I was under the impression the action spring was a 5K round part, although admittedly I can't claim that is or isn't in the TDP. I'll need to read through it some time soon. As you said, timing of the whole system is key, and even though I have other springs that are about the same health in other rifles, obviously those systems as a whole are in "better" shape than the rifle in question.

  5. #5
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    "I'm not locked into the whole brass ejection chart thing".

    Exactly. That chart is just a general reference. I have a rifle that runs like a sewing machine but ejects at 2 o'clock, is what it is. Shooting suppressed like you are is even harder on consumables so tune ups may be a little more frequent. That's why I change out all those parts at one time. Trying to remember when I changed out individual parts drove me insane. I have heard that a rule of thumb on the ejector is if you can press it down with a thumbnail and your nail bends the spring is still good. When you do that and the nail doesn't bend over its time to swap out the spring. Quick field check anyways. You could also have brass shavings impeding the movement of the ejector or carbon build up as Slippers was alluding to I believe. The ejector has constant pressure on the back of the casing and the extractor is just pulling to the right so to speak. So if the ejector spring is weak or the ejector is impeded it will be a little late on ejection causing the extractor to hang on to it a little to long. Basically shit rolls downhill so to speak. That's the way I have always understood it. When you have the bolt apart use a little compressed air to clean out the ejector well.
    Last edited by Stone; 20 October 2018 at 06:19.
    The best way to survive a violent encounter is to be the one inflicting the most violence.

  6. #6
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    Are you getting stove pipes or is the whole case backwards inside the receiver?

    If stove pipes then I'll go with over gassing, but still test your ejector tension since this is easy and quick.

    Do you know what the port was enlarged to when the barrel was chopped?
    Will - Owner of Arisaka LLC - http://www.arisakadefense.com

  7. #7
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    Quote Originally Posted by Stone View Post
    "I'm not locked into the whole brass ejection chart thing".

    Exactly. That chart is just a general reference. I have a rifle that runs like a sewing machine but ejects at 2 o'clock, is what it is. Shooting suppressed like you are is even harder on consumables so tune ups may be a little more frequent. That's why I change out all those parts at one time. Trying to remember when I changed out individual parts drove me insane. I have heard that a rule of thumb on the ejector is if you can press it down with a thumbnail and your nail bends the spring is still good. When you do that and the nail doesn't bend over its time to swap out the spring. Quick field check anyways. You could also have brass shavings impeding the movement of the ejector or carbon build up as Slippers was alluding to I believe. The ejector has constant pressure on the back of the casing and the extractor is just pulling to the right so to speak. So if the ejector spring is weak or the ejector is impeded it will be a little late on ejection causing the extractor to hang on to it a little to long. Basically shit rolls downhill so to speak. That's the way I have always understood it. When you have the bolt apart use a little compressed air to clean out the ejector well.
    Copy all. I'll have a look.

    Quote Originally Posted by Slippers View Post
    Are you getting stove pipes or is the whole case backwards inside the receiver?

    If stove pipes then I'll go with over gassing, but still test your ejector tension since this is easy and quick.

    Do you know what the port was enlarged to when the barrel was chopped?
    It's the case 90 degrees primer facing in/bullet end facing out (pointing to the 3 o'clock) with the next round half-way into the chamber but blocked due to the spent case pushing down and preventing a complete feed. I don't know what the port was enlarged to, but I believe ADCO does it to the Crane spec. When I had the whole thing apart a couple of months ago putting the FF RAS on it, I put it all together and then realized I forgot to check. D'oh!

    I'll play with checking stuff over the next few days. Thanks all.

  8. #8
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    Yeah, that's a stove pipe. Ejector or gassing. Crane spec on a 10.5 should be 0.069-0.070". If you're getting stove pipes without the suppressor then I definitely suspect ejector.
    Will - Owner of Arisaka LLC - http://www.arisakadefense.com

  9. #9
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    Same problem I had on my .308. Didn't happen until I put the can on, and then first double tap, everything went to hell.
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  10. #10
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    Did a detailed cleaning of the bolt and popped out the ejector. Nothing seemed abnormal, before and after reassembly. I blasted the spring channel with some brake cleaner, just in case something was in there. With the ejector installed, it still was stiff, as Stone described (bent finger nail). Forgot to add in the original post that the bolt (and sub-components) only has 2400 rounds on it.

    At this point, I'll just have to take it back out with a spare bolt and see what happens.

  11. #11
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    Only posting this for thoroughness... After the range owner recommended checking for any "gook*" in the extractor, and find said gook* and removing it, the problem disappeared, now after several hundred rounds (both suppressed and unsuppressed). At the end of the day, it was a simple solution that I didn't think i needed to worry about, but I guess it's a touch more sensitive than other uppers with more rounds.
    *Highly technical term for particulate. See also: grime.

  12. #12
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    Yeah those dental picks come in handy for getting down inside the extractor claw...
    The best way to survive a violent encounter is to be the one inflicting the most violence.

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