Having issues with this old Colt upper

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  • Ark

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    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.
     

    Ark

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    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.
     

    Gaffer

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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
     
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    Ark

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    I should be able to get out tomorrow and try it with a known good complete lower and a 1,000ish round known good BCG. Results of that should hopefully tell me what direction to go.
     

    bcannon

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    To answer your questions, a standard rifle buffer weighs 5 oz. Yes they do make lighter ones. Yes you can disassemble the buffer by driving out the roll pin, remove the rubber bump pad and dump out the weights. You can replace one of the weights or all if you want with lighter ones. I would drop your upper on a known fully functional lower like your 6920, easy way to see if its a lower issue. It could be a slightly tight chamber but I'm betting that its a slightly clogged gas port or tube. I'll bet 4 dbl bacon cheeseburgers on it. I picked up a new/old stock upper for cheap that had similar issues. Put a small hose on the tube in the upper and could blow thru it but it took disassembling it to find thickened oily substance in the tube and gas block. Cleaned with a 12ga wire and a pipe cleaner, reassembled and had no issues after that. Good luck finding the issue.
     

    Ark

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    Swapped lowers and tested. Rifle upper runs fine on the carbine lower, carbine upper runs fine on the rifle lower with one failure to lock open.

    Looking like an issue with the lower. The carbine upper is gassed more and can run on it, but the rifle upper just can't. The rifle upper can run on a carbine buffer and spring.

    I think the next step is to try to lose about 2oz from the rifle buffer. I saw some people have replaced the weights with a section of aluminum rod to retain at least some anti-bolt bounce capability. I may also pull the gas tube and clean it out with some wire or something just to be sure.
     

    SmileDocHill

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    Any idea as to whether it has been used with a .22 conversion kit? That will support junk accumulation in the gas tube more than normal. It's my understanding the gas tube historically is not otherwise prone to buildup. (?) Not that it can't happen, not that I wouldn't check that as a possible source of the bottleneck, just saying for conversation sake so other more informed members can correct my logic.
     

    Ark

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    Six weights total, all weigh out at .64 oz, indicating steel and not tungsten.

    Aluminum weights are about .22 oz.

    .42 oz weight savings per weight changed, maximum I can remove is 2.52 oz without taking weights out altogether. That should reduce the rifle buffer weight to at or below the weight of a standard carbine buffer.

    Any idea as to whether it has been used with a .22 conversion kit? That will support junk accumulation in the gas tube more than normal. It's my understanding the gas tube historically is not otherwise prone to buildup. (?) Not that it can't happen, not that I wouldn't check that as a possible source of the bottleneck, just saying for conversation sake so other more informed members can correct my logic.

    I kinda doubt it. The upper is LE surplus and was probably dumped in storage long before those CMMG conversions became a thing, But, it seemed to run great on the carbine lower even on a dog's lunch magazine of low pressure steel case.
     
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