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

  1. #1
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    Colt with 11.5" barrel failing to lock back on last round

    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.
    Last edited by masakari; 12-19-15 at 12:37.

  2. #2
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    Sounds like it's short stroking, do you have a h buffer you could try?

  3. #3
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    Tme obvious place to start is with some different mags.
    Lucius Quinctius Cincinnatus (519 BC – 430 BC) Power should only be given to those that want it least.

  4. #4
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    Quote Originally Posted by M&P15T View Post
    Tme obvious place to start is with some different mags.
    I should note that magazines and ammo are not subject to change.

  5. #5
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    Quote Originally Posted by rushca01 View Post
    Sounds like it's short stroking, do you have a h buffer you could try?
    Thing is, it is feeding 100% when shooting. No failures to feed.
    I only have an H3 buffer now.
    Last edited by masakari; 12-19-15 at 12:30.

  6. #6
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    Yes, your rifle is now slightly under gassed when using certain types of ammo. A lighter buffer will probably fix that and you'll be able to go back to a heavier buffer when the suppressor is installed. I think your gas port is right where it needs to be
    INSIDE PLAN OF BOX
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  7. #7
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    You should have switched to a lighter buffer, not a heavier one.

  8. #8
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    Sounds all good to me. Id rather my sbr be short stroking with an H3 and no suppressor than just about any other problem I can think of. As others have said, don't touch the gas port. Id go ahead and purchase milspec (no spikes powder stuff) H and H2 buffers. I bet running the H2 buffer unsuppressed will fix the short stroking, but if it doesn't, the H buffer will almost certainly straighten things out. Then, when you do end up putting a suppressor on there, you can throw the H3 buffer back in there. Go ahead and pick up a milspec carbine action spring from bcm while you are at it (just to make sure your action spring isn't a bit weak and contributing to the problem). Let us know and, for god's sake, don't touch that gas port.

  9. #9
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    Colt with 11.5" barrel failing to lock back on last round

    Hopefully IG chimes in, but I think the .073 gas port size is more for the 10.3/10.5" barrel. I believe the 11.5" should be between .080-.090 range. Keep in mind my info is coming from Google, so...

    http://www.tacticalmachining.com/lea...ort-sizes.html
    Last edited by Inkslinger; 12-19-15 at 15:15.

  10. #10
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    Agreed with all points. Try going back to an H buffer and see if that resolves your issues. You can then move to an H2 and test.
    Best Regards,
    28_days

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