As others have said you CAN measure BP by weight since theoretically 40 grains by volume from your powder measure should weigh 40 grains. When loading cartridges I weigh my charges of BP. More specifically, if I want 40 grains of BP I calibrate my Lee powder measure to throw 40 grains of BP by weight and verify it on a scale. Obviously, the Lee powder measure isn't weighing the powder but rather is volumetrically throwing the powder.
