Loose black powder is not meant to be measured with a scale. It is supposed to be more consistent, in terms of pressure and velocity, when measured by volume; and there is no reason to even attempt a conversion. Only the folks shooting smokeless in the Savage guns need a scale.

Also, they do make 30 grain pellets in the .50 caliber. I think Cabela's has them. A number of gun stores around here carry them as well. Two fifties and a thirty - great load for 250 or 300 grain SST's/Shockwaves. Good luck.