Does anyone happen to have a spare carbine tube they could measure to see how long it is?
Update in the OP.
Read your update. IMO something is still wacked with that upper. You should be able to put any reasonably well built BCG into any reasonably well built upper. Is the gas tube still protruding into the upper farther than some other uppers you've inspected? I did a quick measure comparison of 4 gas keys and they were all different internally to the internal shelf. They were all within about an 1/8" of each other. I could interchange them all.
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