Only time you will get velocity drop is if the barrel is ported which in effect shortens the effective length of your barrel and in my opinion, a very poor way to reduce recoil. The Holland is one of the best out there for reducing recoil. You will be happy, your 300 RUM will recoil about like a 25-06 at the most.
Allen Precision Shooting
Home of the Allen Magnum, Allen Xpress and Allen Tactical Wildcats and the Painkiller Muzzle brakes.