Because there are no universal standards for volumetric grains, never has been. Black powder and its substitutes are meaured by weight, not volume. As is stated on the bottle....10 oz, for example. Measuring by volume is not precise. It can measure all over the place by geometry, moisture content and composition. Not only does volumetric grain have no correlation to weight, it has no precise correlation to performance. Not only that, volume differs by propellant. 100 grains by volume of BH, is not the same weight of 100 grains of Pyrodex, and so on,
With all that being said, if it works for you...do it. I have always used pellets, which are supposedly the most inconsistent of all. But i go to the range, sight in, and then go kill deer. I have just wanted to try this route, being motivated by some of you fine gentleman and seeing your results posted. Also, im a little obsessive compulsive, lol. If im gonna do it, i wanna do it right.
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