It is not measured by volume, that is just a convenience that a set volume weighed x amount since the powders are consistent.
While technically grains are a weight measurement, volumetric grains are a measurement of volume equivalents used for all of the different muzzle loading powders.
Since black powder can come in so many different densities, IMHO the volumetric measure has established a standard of its own.
For instance, the volume and weight of water and Goex black powder is virtually identical.
So I personally believe that a volume of grains of water has been arbitrarily established as being the standard used for manufacturing volumetric black powder measures.
And that's also why all reference to the grain charges of black powder are understood as being by volume rather than by weight unless specifically mentioned as being a weighed charge.
I think that it's a very peculiar volumetric method of measurement that has been developed strictly for the muzzle loading community over recent generations by the muzzle loading industry.
They know that home made and commercial black powders and substitute powders will all have different weights and densities for equal volumes.
But the users & manufacturers need to have a volumetric standard and most measures seem to be standardized enough to be based on the weight of a volume of water.
Sure they could use Goex but which granulation and how compacted should it be?
Whereas the density of water is pretty much accepted as being universal everywhere in the world.
And that's also the reason why the cartridge case capacity in grains of water [relatively] accurately reflects the volume of black powder that each case can hold and can be used as an measure of.
While it may be a coincidence, all cartridge cases have a case capacity that's usually always measured in grains of water.
It's a known volume with a known weight which is basically identical to Goex FFFg.
While each batch of even Goex can have a different density, I believe that the volume of powder measures has been pretty much standardized by the manufacturers.
They can measure the volume of 100 grains of water and design and build their measures to be an acceptable enough standard for use around the world with every [commercial] powder.
Do cartridge case capacities vary according to brand or batch? Yes, but generally within acceptable enough limits to not matter unless a person wants to weigh their charges and/or use the converted weights.
Revised Conversion sheet --->>>
https://www.curtrich.com/BPConversionSheet.htm
The original conversion chart showed that Goex ffg equaled 100 grains by vclume using an adjustable powder measure instead of CC scoops. --->>>
https://www.curtrich.com/BPConversionSheet2005.htm
How do they make the CC scoops? I believe that they make them based on the volume of a known weight of water.
1 CC [cubic centimeter] of water equals 1 gram.
