Originally Posted by
Smokeless Coal
Think of it this way, if you were to fill your 20gr(vol) spout with fine lead shot do you think it would weigh the same as the same volume of BP? of course it wouldn't.
All powders have a different weight per volume, this includes some BP. The chart Roy linked to shows the comparisons, its not perfect but it helps.
The guns are not fussy about slightly higher or lower loads, but variation is not good if you are trying to improve accuracy.
For my own loads I use the powder dispenser I use for my cartridge reloading, volmetricaly dispensed (17.5gr by vol) but then I double check and weigh each load.