Yes, blackpowder is always measured by volume instead of by weight. pahoo is correct in saying the bp and the subs are very forgiving and volume measurments are really convenient in the field. If you want to get into the density of the subs versus real bp, and the real technical reasons for measuring by volume, there are guys on here that can explain in detail. If you really want to dig into it, google "ffg to fffg conversion" and you'll find some really detailed threads about volume to weight conversions.
Beware the man with one gun, he knows how to use it