Welcome to the forum from New York.
The 300 Winchester Magnum has a rim diameter of .532 and can be used in a standard long action rather than a magnum action like the H&H Magnum. The 7mm Rem Mag has the same base dimension so the bolt face will work. The 300 is a longer case. I assume your 300 is not on a magnum action. Does the bolt physically fit?
Bob