From another site:
According to AMU (Army Marksmanship Unit) specs, do not torque the flash hider. Get yourself some red Loctite and leave off the crush washer. Hand tighten the FH until properly timed, then allow to "set up" overnight. Torquing the FH onto the barrel stretches the barrel threads and screws up your barrel harmonics.
I see 15-20Lbs and also 35-40 Lbs as well- a confusing array of information.