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  1. #1
    Ark
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    Having issues with this old Colt upper

    I'm hoping there's some people around INGO who may have firsthand experience with these guns and anything wonky or special about them.

    https://imgur.com/a/6ChAUll

    This is a complete album with a lot of detailed pictures.

    The upper is a Colt Match Target MT6551 from sometime in the mid-90s. It is a police surplus upper that popped up in a one-time sale at Arms Unlimited a couple weeks ago (falsely advertised as having a threaded barrel and A2 flash hider, but that's another story). It is a 20" barrel, chrome lined, rifle length gas, unthreaded. The bolt carrier is an original Colt BCG, Colt proof marked, with the ban era cutout to prevent auto sear use. The gas key is staked properly, and both the gas key and gas tube appear unobstructed.

    The lower is a factory new PSA complete lower. Rifle length buffer tube, rifle spring, A2 rifle buffer, and a Larue MBT2S. Magazines were known good PMAGS.

    The issue here is that the rifle appears very undergassed and is short-stroking. Test ammo was Barnul 55 grain and 62 grain, Wolf 55 grain, and IMI 55 grain XM193. The steel case ammo completely failed to cycle. Half the time the bolt would not pick up another round, the other half of the time the bolt would overrun the cartridge and jam it as pictured. A failure occurred on every round fired. The pictures show the bolt over the top of the round, not catching the back of the case. The steel case ammo completely failed to engage the bolt hold open. The bolt held open with the PMAGs if it was pulled back by the charging handle, so doesn't look like a mag or lower issue.

    The IMI ammo fared a little better. No bolt overruns, and it would cycle, but it only engaged the LRBHO about half the time. All ammo extracted fine and ejected at 4 o'clock.

    I have, frankly, no use for a rifle that won't run steel case ammo at all, and it seems barely gassed enough with fully hot 5.56. Clearly there's an issue, either with anemic gassing on the upper or a spring/buffer problem on the lower. I can blow air through the gas port and tube and it does not feel obstructed. The rifle was field stripped, cleaned, and oiled thoroughly before testing.

    A4 rifle buffers are around 5 oz, which is on the heavy side. What buffer would have been matched to this upper on the original Colt-built rifles? Are there lightweight rifle buffers available that could mitigate this problem? Can an A4 rifle buffer be disassembled and some amount of weight removed? Or, should I be getting a spacer and ditching the rifle buffer/spring altogether and converting to carbine?

    I had anticipated this configuration running great, maybe even being overgassed due to the missing mass on the BCG. The opposite seems true. Tomorrow I may fabricate a spacer and swap one of my carbine springs and buffers into it and testing it again, or just drop it on my 6920 lower to see if it works.

  2. #2
    Grandmaster phylodog's Avatar

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    Your issue has the classic symptoms of a loose gas carrier key. Have you checked that?
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  3. #3
    Ark
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    Quote Originally Posted by phylodog View Post
    Your issue has the classic symptoms of a loose gas carrier key. Have you checked that?
    Carrier key had no discernable play, and the screws were well staked. I did not see any unusual carbon buildup on the carrier to indicate gas leakage, or around the pinned FSB.

    Swapping in the BCG from my 6920 would probably also be a useful diagnostic step. I was dumb to not bring that rifle to the range with me.

  4. #4
    Dented Helmet Gaffer's Avatar

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    A bit hard to tell, and it might be just something in the photo but the end of the gas tube looks eroded .. not even sure if that would make any difference or not.

    I have a 20" A2-ish, and it will not shoot any of my reloads that are lighter loads. It does what yours does if they are too light of load. All my other AR's cycle them fine.

    Ron
    Last edited by Gaffer; 12-12-2019 at 17:24.
    “We do not rise to the level of our expectations. We fall to the level of our training.” -Archilochus

  5. #5
    Marksman

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    Have you checked the gas block for alignment?

  6. #6
    Ark
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    Quote Originally Posted by edwea View Post
    Have you checked the gas block for alignment?
    Hits dead on to the sights at 50 yards, two clicks from center on the rear sight. FSB is pinned from the Colt factory.

  7. #7
    Grandmaster Vigilant's Avatar

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    Also test the gas rings on the bolt, short stroking and FtF’s can be caused by worn rings at times.

  8. #8
    Master

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    Is the bolt 90° out of sync?
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  9. #9
    I still care....Really
    churchmouse's Avatar

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    Quote Originally Posted by Vigilant View Post
    Also test the gas rings on the bolt, short stroking and FtF’s can be caused by worn rings at times.
    This. Pull the BCG all the way down and clean/check the bore they run in.

    Swap bolts. Bet it runs.
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  10. #10
    Midnight Rider

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    A lot smarter guys than me in here. As CM mentioned, I've seen swapping bolts "fix" a few guns.

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