I'm sure to be corrected if I'm wrong, but speaking about Black Powder being measured by volume, your absolutly right. That is the way we measure our charges.
There is actually a direct relationship here.
If you fill a 70 grain powder measure with Real FFg Black powder and then weigh it, it will be very close to 70 grains weight.
If you use FFFg, it will be slightly more.
In other words, if your measuring Real FFg Black Powder the weight and volume go hand in hand.
If everyone shot FFg Black Powder, we wouldn't have to answer a lot of posts about this volume/weight relationship.
The problem comes with powders like Pyrodex, 777, etc.
These powders are meant to be measured and used using the Black Powder Volume method but per cubic inch, they don't weigh the same amount as Black Powder.
This causes some folks to pour things like Pyrodex into a 70 grain powder measure and then they weigh it.
They get answers like 47 grains. Then they get to thinking their measuer is the wrong size.
What's happening here is that Pyrodex only weighs 2/3 of the weight of FFg Black Powder per cubic inch volume.
Pyrodex even makes the claim that you get 30% more shots per pound of powder than if you use black powder.
The reason for this is 47 grains of Pyrodex not only has the
same volume as 70 grains of Black Powder, it also has the
same amount of POWER as 70 grains of FFg, so if your loading one of the synthetic powders, just go by the old Black Powder Volume method and you'll be fine.