A brake on a 300Win Mag will cause hearing damage. Ear plugs AND muffs are highly reccomended. If the brake has any holes except pointing downrange (linear brake) you will expierance an elevated db level . . AND . . an elevated concussion/overpressure level.
Most,if not all brakes that actually work,and work good,are not the cool looking ones. IMO the two that are the very best are the JP Recoil Eliminator and the Precision Armament M4-72.
I have both . . . they are loud . . . BUT THEY WORK. --- SAWMAN
ADDED: Your rifle will need to be threaded 5/8X24.