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Expert Answer
The buffer assembly that I would recommend for this upper receiver is the Aero Precision M5 Buffer Kit. It was specifically designed to work with the M5 lower and upper. It includes the 308 buffer spring, 308 carbine buffer, end plate and lock nut, and a milspec buffer tube. You can find the Aero Precision Buffer Kit using the following link:
https://www.opticsplanet.com/aero-precision-m5-308-carbine-buffer-kit.html