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

  1. #11
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    It's undergassed. An 11.5" should be around .076. I think I mentioned this when you contemplated sending it out for the Caitlyn remake. If you are shooting it suppressed, that size will be fine even with .223.



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  2. #12
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    Incorrect and those numbers are mostly crap.

    Quote Originally Posted by Inkslinger View Post
    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



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  3. #13
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    Quote Originally Posted by Iraqgunz View Post
    Incorrect and those numbers are mostly crap.
    Yeah, I figured. Most info you find online is all over the map.

  4. #14
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    If it were my gun I would switch it to an A5 system and start with an A5H2 buffer.
    Steve

  5. #15
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    Just to clarify. You originally mentioned cutting this to 11" with the ammo you were talking about. Did you actually do that or did you do 11.5"?

    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.



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  6. #16
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    Quote Originally Posted by Iraqgunz View Post
    Just to clarify. You originally mentioned cutting this to 11" with the ammo you were talking about. Did you actually do that or did you do 11.5"?
    I could have sworn you told me .073. Well that's probably the issue.
    .076 then? Would you stick with the H3 buffer?
    The barrel was cut so that the flash hider butts up against right where the m203 notch would normally start. I believe it's very close to 11.5".

    Edit: just checked, you had previously told me .076, then corrected it to .073. But I can see about having it opened.
    Last edited by masakari; 12-19-15 at 17:07.

  7. #17
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    I think the confusion was coming from the various other postings.

    A 10.5" with a .071 port will work fine with MILSPEC (not .223) ammo suppressed and unsuppressed.

    An 11.5" with a .076 port will work with 5.56 and most .223 ammo. Providing you don't over buffer it. Your .073 should work fine with .223 and the suppressor. Without the suppressor an H buffer and most likely an H2. Not an ST-2, an H2.

    Quote Originally Posted by masakari View Post
    I could have sworn you told me .073. Well that's probably the issue.
    .076 then? Would you stick with the H3 buffer?
    The barrel was cut so that the flash hider butts up against right where the m203 notch would normally start. I believe it's very close to 11.5".

    Edit: just checked, you had previously told me .076, then corrected it to .073. But I can see about having it opened.



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  8. #18
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    Quote Originally Posted by Iraqgunz View Post
    I think the confusion was coming from the various other postings.

    A 10.5" with a .071 port will work fine with MILSPEC (not .223) ammo suppressed and unsuppressed.

    An 11.5" with a .076 port will work with 5.56 and most .223 ammo. Providing you don't over buffer it. Your .073 should work fine with .223 and the suppressor. Without the suppressor an H buffer and most likely an H2. Not an ST-2, an H2.
    Thanks. So, my options are:

    Keep it at .073", H1 or maybe H2 buffer
    Drill to .076", H2 or H3 buffer

    Correct? Sorry, I'm just trying to plan things out. I already dropped the upper off to get drilled or tested with a new buffer. Busy schedule coming up, and all I have is the H3 to test with now. And of course, all the local shops are out of everything.
    Last edited by masakari; 12-19-15 at 20:10.

  9. #19
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    If the port is opened to .076 which is the same size as my BCM 11.5" and the same size as the standard SIONICS it will work suppressed with an H3. In fact, before switching to the A5 several years back, my BCM 11.5" wouldn't lock back consistently with a suppressor and H2 buffer regardless of ammo. So I bumped it to the H3. Shortly afterwards went to the A5H4.

    Quote Originally Posted by masakari View Post
    Thanks. So, my options are:

    Keep it at .073", H1 or maybe H2 buffer
    Drill to .076", H2 or H3 buffer

    Correct? Sorry, I'm just trying to plan things out. I already dropped the upper off to get drilled or tested with a new buffer. Busy schedule coming up.



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  10. #20
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    The change in function between a .073" port to a .076" isn't large on a 11.5" 5.56.
    With that port and base barrel, you should probably run an H2 or A5H2 with 5.56 pressure ammo without a can. You could step up to a heavier buffer with a can.
    I personally would't ream the extra .003", I tend to like many 11.5" guns with a slightly smaller port than that.
    Depending on ammunition with a standard carrier, an A5H3 may be more preferable without a can, depending on what that ammo is. The powder buffers do not perform as reliably in the M16 FOW as others.

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