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Grains is a unit of weight, and it is used that way in measuring smokeless powder, so it's use as a volume measure in black powder shooting can be confusing.
It works like this: for real black powder the volume of a certain weight of powder in grains is said to have the same number of grains of volume. For example, 35 grains by weight of black powder takes up a certain volume; that volume is said to be 35 grains by volume.
The use of volume for black powder has to do with the ease of volume measurement as compared with weight measurement when in the field.
However, this relationship between weight and volume measurement only applies to real black powder. Synthetic or substitute black powders have different densities than real black powder, so 35 grains by volume of Pyrodex does not weigh 35 grains, it weighs less. However, 35 grains by volume of Pyrodex gives about the same pressure wave as 35 grains by volume of real black powder; it just weighs less.
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