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Thread: Colt with 11.5" barrel failing to lock back on last round

  1. #21
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    IMO, the OP should just drop down to an H buffer and be done.

    One of the main selling points of the 11.5" is the increased "power time" for the gas system.

    IF a 10.5" barrel runs well with an .071 port, Given the same ammo and buffer, Why would a 11.5" barrel need a port any larger?
    Last edited by Clint; 12-20-15 at 10:03.
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  2. #22
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    With an unsupressed 11.5" with a port of .073" I'd start with an H buffer.
    Chief Armorer for Elite Shooting Sports in Manassas VA
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  3. #23
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    Quote Originally Posted by masakari View Post
    I'm having a problem where my 11.5" barreled Colt duty rifle is not locking back reliably on the last round. Sometimes it locks back properly, but sometimes it locks back halfway, with the bolt stop on the carrier instead of the bolt face.
    There have been no other issues or failures whatsoever.
    Details:

    Rifle: Colt 6940, barrel chopped to 11.5" by ADCO, gas port should be .073.
    Rifle has seen a total of maybe 1000 rounds, and is in excellent condition. Gas rings not aligned, gas key not loose. Properly cleaned and lubricated. Lock back issue did not exist when the barrel was 16".

    Magazines: Multiple Lancer L5AWMs in working condition.

    Buffers: Started with Spike's ST-T2, changed to H3 on recommendations. Did not solve issue, MIGHT have made it slightly better.
    Using stock Colt spring.

    Ammo: Tested with M855, Federal Champion .223, and Federal .223 T223E duty ammo.

    Problems: Rifle has fed ammo just fine while firing, no short stroking with any ammo, even during rapid fire.
    The issue is that with the .223 ammo, the bolt fails to lock back on an empty magazine almost every time (seems slightly better since I switched buffers from ST-T2 to H3, but I'm not sure). Locks back almost every time with M855, but not always.
    When it fails to lock back, the bolt is NOT going all the way forward on an empty chamber, rather, it is locking halfway back with the bolt stop resting on the carrier instead of the bolt face. So far, through hundreds of rounds, this is the only type of failure that I have experienced.
    Rifle ejects to the 3:00 position without much force, brass goes about 2-4 feet.

    I'm thinking that out is slightly under gassed. The gas port was supposed to be .073, but I have no way of confirming.
    Rifle has not been shot suppressed, but it will be later when I get a suppressor, so I don't want to open the gas port more than I have to.

    Please let me know what you think.

    I think I found your problem. Why didn't you buy a 6943 upper or complete gun?
    "Not every thing on Earth requires an aftermarket upgrade." demigod/markm

  4. #24
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    Well, we found the culprit: gas port is ACTUALLY at .062. You see, ADCO chopped the barrel, but never actually opened the gas port as I had asked and paid for, so it's currently stock as it would be at 16".
    So, a component gunsmith here locally is opening it to .072 to start with. I will test and go from there.
    I'm also buying a carbine buffer so that if need be, I can swap weights around to make any type of buffer.
    Last edited by masakari; 12-21-15 at 20:33.

  5. #25
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    Just an FYI. I think you are overthinking this. We did some pretty extensive testing with various port sizes. If you are going to run this suppressed a .072 will be adequate. If you are going to run .223 ammo it will also be fine. If you remove the suppressor there is no guarantee. I would simply port it at the KNOWN size of .076 and then run an H3 buffer and be done with it.

    Quote Originally Posted by masakari View Post
    Well, I found the culprit: gas port is ACTUALLY at .062. You see, ADCO chopped the barrel, but never actually opened the gas port as I had asked and paid for, so it's currently stock as it would be at 16".
    So, a component gunsmith here locally is opening it to .072 to start with. I will test and go from there.
    I'm also buying a carbine buffer so that if need be, I can swap weights around to make any type of buffer.



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  6. #26
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    Quote Originally Posted by Iraqgunz View Post
    Just an FYI. I think you are overthinking this. We did some pretty extensive testing with various port sizes. If you are going to run this suppressed a .072 will be adequate. If you are going to run .223 ammo it will also be fine. If you remove the suppressor there is no guarantee. I would simply port it at the KNOWN size of .076 and then run an H3 buffer and be done with it.
    I very much value all your help.
    That may be what happens. Since this guy is so close, he recommended that we start at .072 and if needed, open it up to .076 after. His reasoning was that it cycled perfectly so far at .062, my only issue being that it wouldn't lockback with .223. So it seems like I'm relatively close. But I'll test it tomorrow, and go from there. I'll update here with what happens.
    I'm just disappointed that ADCO didn't do what I asked originally.

  7. #27
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    Quote Originally Posted by masakari View Post
    ............I'm just disappointed that ADCO didn't do what I asked originally.
    I'll be blunt..

    ADCO can't do jack! And when they are questioned about their crappy work, they scratch there heads and say there isn't a problem.

    They need their own "Gunsmithing fail " thread.

    ....ask me how i know.....

  8. #28
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    Before making the jump to a gas port that may be larger than needed What is the .223 ammo?
    Stepping up to .067" port may be a plausible solution with a H2 without excess gas, ammo depending.
    The OP states that it seems close, it may be so?
    I tend to prefer operation without suppression using less than a H3 in a carbine action. That way additional weight may be added with a can, depending on its attributes.

  9. #29
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    Off the top of my head, if he ran it suppressed all the time a .067-.069 would be fine. If he removes the suppressor it will be ammo dependent for sure. I prefer a little more gas to be safe this way it will run with other ammo should I need it to.

    Quote Originally Posted by tom12.7 View Post
    Before making the jump to a gas port that may be larger than needed What is the .223 ammo?
    Stepping up to .067" port may be a plausible solution with a H2 without excess gas, ammo depending.
    The OP states that it seems close, it may be so?
    I tend to prefer operation without suppression using less than a H3 in a carbine action. That way additional weight may be added with a can, depending on its attributes.



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  10. #30
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    If it was dedicated suppressed, it more than likely that it doesn't even to need to as large as .067". Depends on the attributes of the can and ammo. Many run dedicated suppressed 11.5" guns with the .062/.063" port.
    What is the .223 ammo used? That can make a big difference.

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