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Can anyone help me understand the "equivalent volume of blackpowder" concept? I don't have and from what I'm lead to believe, can't actually buy real blackpowder so I can see what volume 12-15 grains occupies to match it with Pyrodex.
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Volume equivalent is what a volumetric powder measure does measure. It measures a volume of black powder and subsititute powders in volumetric grains as opposed to their weight in grains.
In theory, 100 grains of BP by weight equals 100 grains of BP by volume.
Substitute powders like Pyrodex occupy the same volume but don't weigh the same.
That's why whenever amounts of muzzle loading powders are being discussed, it's always in terms of volume and not weight (unless weight is specifically mentioned as the unit of measurement).
There are slight volume differences between different granulations of powder, ffg, fffg, Pyrodex RS or Pyrodex P (pistol) because of the amount of air space between the granules. That difference is approximately 10% that a volume of fffg (or P) would need to be reduced to match an equivalent volume of ffg (or RS).
Thus, 100 grains volume of ffg equals 90 grains of volume of fffg (or Pyrodex P)
Loose 777 should always be reduced by 15% in volume to be equivalent to any volume of BP or other substitute powder.
Thus, 100 grains volume of ffg BP equals 85 grains of volume of 777 ffg.
Otherwise, in theory any volume of BP and BP substitute powders are volumetric equivalents. In reality there are velocity differences. But the pressures are supposed to be pretty equal which make them safe to use interchangably.