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Hop-Up is in working condition and the O-Ring on the piston head is making what appears to be a good seal with the cylinder walls.
I'm fairly familiar with the internals of the MP5 series as I have owned 5 of them through out my career. Also, just for information more than anything, I have taken it to a gun doctor and he has no idea what's going on either. Everything "appears" to be fine.
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