I've always thought that real black powder is best measured in volume and that measuring any other way is a waste of time. The granulation of black powder is so irregular and contains so many fines that weighing would just turn out to be very inconsistent while volume is volume is volume.
Respectfully offered, volume is volume only if variables are eliminated.
Let's take a look: have you ever tested the volume of BP thrown by a handful of powder measures? You'll be surprised, because the 100 gr. marks on measures will throw weights of the
same powder differing by as much as 5 grains. That's no big deal for ordinary fun shooting. But not if you are shooting longer range or match-shooting. Then, use the same measure every time.
Next, the brands of powder. Some are denser, like Swiss, whereas some are lighter, like KIK. So the
bulk may be the same from a given measure, but the "oomph" will vary by how the powder was made and just what screen size the maker chose to market and sell as "2F", "3F" or whatever. Thus 100 gr. volume of one powder that is "hotter", like Swiss, might provide more pressure and velocity and thus spin your longer bullet just enough in the twist rate of a given barrel to stabilize that bullet, whereas 100 gr. volume of another that is less powerful might fail to do so. The bullet strikes on the target will vary also, because of differing "barrel time" for the fired bullet, as well as gravity.
Then there is the fact that a can of powder does not contain an even granularity of powder like you may find in a smokeless powder can. This is especially true with BP that has been shipped all around, or carried a good ways by highway, or a shorter ways on a rough road, in a vehicle or trailer. The "fines" may have settled to the bottom whereas the larger grains are at the top. And the finer grains gather in any container as a lesser volume, yet burn faster and provide more power. So for consistency, like match shooting, it's not uncommon to see me or somebody else repeatedly turning our powder can upside down or twisting it all around, to spread the finer grains all around in the can before we begin to load. Otherwise, you get different performance from your measured powder the further down in the can you are pouring from. This is more important for pistol shooters using lesser amounts of powder, tho.
Lastly, take a given powder measure, and weigh out, say, 100 gr. by volume of black powders of different makers, plus substitute powders. Stick with the same granularity tho, like 3F or 2 F. You will find that the volume vs the weight varies all over the place this way. That's why it is always important, when speaking of muzzleloader or cartridge loads using these powders, to always designate the powder and its granularity size, and especially either weight or volume. With the more powerful powders, like Blackhorn 209, Swiss and Triple 7, this is distinctly important.
Aloha, Ka'imiloa
