There is no such thing as "grain volume, volumetric grain, volumetric equivalent", etc.
Grains are a measurement of weight, or mass.
Volume is the amount of space in a three-dimensional object.
The amount of powder dropped by a powder measure (which has a three-dimensional chamber of a particular volume) will have a particular weight (as measured on a scale). However, if you switch from Fg powder to FFFg powder without changing the powder measure, you will certainly drop a different weight from the same volume. Even if you switch from brand to brand, you may drop different weight. Two different powders occupying the same volume may not have the same mass. Two different powders having the same mass may not have the same volume.
If you need to fill a given volume (such as black powder in a cartridge or shotgun shell), fill the volume and don't worry about the weight or mass.
If you need a particular weight (such as smokeless powder in a cartridge or shotgun shell), use a scale and don't worry about the volume.