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Posted: 11/9/2011 2:32:22 PM EDT
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I've bought an upper built by a reputable manufacturer about 4 or 5 years ago. I splurged on the thing and bought the best parts offered. So imagine my chagrin when the fucking thing wouldn't function to save my life while my $700 Bushmaster functioned like a champ. When it got hot from firing maybe 10-15 rounds the upper stopped cycling.
Failures to feed, failures to extract, failures to eject- like the bolt group didn't have enough inertia to work. Something was impinging on it, I knew that much. I never could diagnose the problem. I changed out parts in a desperate bid to salvage my investment. No dice on anything I tried. I now realize my mistake was assuming everything was within spec. I mean, I payed a ton of money, it had to be! I was wrong. I discovered while breaking the upper apart to scavenge for parts that the barrel extension is not aligned with the gas port. It's not even close considering the tolerances of the AR. The pin that centers the barrel in the receiver is not in line with the gas port. I would have never figured it out without breaking the upper apart. Since the gas port didn't align with the rifle's longitudinal axis, the gas tube wasn't in perfect alignment with the receiver port- in order to fit it had to be strained and would not behave normally during firing. The carrier key strikes the gas tube and gets impinged. Sometimes it drags off the tube on extraction, sometimes it hits it trying to lock back up. Heating shifts the incorrect tolerances even more. Viola! Shit piece upper. I'm actually kind of ecstatic I figured it out. Just another excuse to buy more armoring tools! 
Maybe this will serve as a heads up to others though. Sometimes the problem really is that simple, and it's one you can't see without stripping the thing down to its constituent components. ETA: Upon inspection of the gas tube, it is all chewed up on one side from the carrier key. |
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You can usually see his type of problem by taking the carrier without the bolt attached and letting it slide home with the upper tilted at 45 degrees. If the tube is out of alignment the gas key and tube will bind up. If alignment is good there will be almost no friction or drag when the key meets the tube. You can also simply slide the carrier back and forth...there should be almost no drag when the key and tube connect.
Glad you found the problem |
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