Is the muzzle brake screwed on / soldered on ?????
If you have a bench vise: Remove magazine. Pull back the charging bolt & visually check chamber to insure the weapon is unloaded. Clamp muzzle brake in padded vise jaws. Turn rifle counter-clockwise in an attempt to unscrew unless it is a right hand thread. If so, then turn rifle in the opposite direction.
If brake is soldered on: Use a propane / acetylene torch to heat brake enough to break the solder bond, while the brake is clamped in the vise.