Grains are avoirdupois units of weight and nothing but.
You can do ok loading black powder by volume but calling the number on the volumetric measure "grains" does not make it so.
Pyrodex is meant to be comparable to black on a volume basis but is about 70% the density so the actual charge weight is less. If anybody is talking about reducing the Pyrodex load 10% from the normal volume of black, he is very timid.
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