As for using it in a 300 Weatherby, that's a bit of a wild card. The 300 Weatherby is a high-performance cartridge that typically requires a fast-burning powder to reach its full potential. H50BMG is definitely on the slow side, so it may not be the best choice. That said, if you're feeling adventurous and willing to experiment, you could try starting with a low charge and working your way up cautiously, monitoring for signs of overpressure. But proceed with caution, and if in doubt, consult a professional reloader or the powder manufacturer for guidance.