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I own one in 16 inch. Bought just the upper and disregarded their buffer/spring advice from day 1. Have the Macon buffer and carbine spring. They do a weird thing where they recommend a 308 spring that changed the timing of the gun then get you into their proprietary length/proprietary weight solid buffer to work properly with that. Upper itself seems fine but tuning recommendations Seem more geared towards selling you their parts than running the gun optimally.
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Not having a good time with my 1st FM upper either. Thankfully, not a kaboom. Glad OP is ok. I have been emailing with them & they clearly did not read what I sent & started trying to sell me their buffers & 308 spring. Unreal.
I've had enough of that so I asked them if I could send it in so they can diagnose & resolve the issue. If not, buy it back as I cannot return it to the vendor I bought it from now.
Here is my thread: https://www.ar15.com/forums/AR-15/Problems-with-Foxtrot-Mike-9mm-build-/4-767338/