Unfortunately, that's impossible to answer!
The only ones that know that are Winchester, and even if you could find the recipe, the powder they use is not available to handloaders.
They use a 'bulk' powder that is blended in the laboratory to give the desired ballistics, which is often different to previous loads in a batch.
You can always try to duplicate the velocity, and the nearest powder we have at our disposal is Win Supreme 780. This is very similar to what is loaded in factory loads for the 300.
Good luck with it, and enjoy!