Originally Posted by Max Heat
I think what threw me was that the back powder era had transformed into [what I thought was] the "water" era, after smokeless powder took over. By that I mean that smokeless powders are typically very simiar to water (1.0), in terms of their density. Case "capacity" actually IS measured that way in modern times. But that sounds like a recipe for a mix-up right there.
I believe this is another area that is confusing you. Water is used to measure case volume because it takes the shape of the container. Measuring case volume by filling the case with water and re-weighing the case allows a consistent and useful basis for comparing the case capacity of different brands of brass and different chamberings.
Water capacity in grains does not directly translate to powder capacity. It is used solely as a means of volumetric comparison.
Different smokeless powders have different bulk densities, meaning that the same weight of powder takes up a different amount of volume depending on powder type. Generally speaking, slower powders will take up more volume versus their weight in grains. Ball powders will typically provide greater load densities (case fill) than an equivalent extruded powder.
As an illustration of the effects of powder bulk density, pour 45 grains of H4350 into a .308 case. Then, try doing the same thing with 45 grains of Trail Boss.