Grains of water includes the case neck. Neck lengths vary even across cartridges of the same caliber, but as total capacity goes up, the percentage of that capacity taken up by the neck decreases quite a bit. Capacities in the table are shown in cubic inches, and came from
this.
I used 6mm GT capacity for the .22 GT line -- have yet to reverse engineer his work on that so I can arbitrarily include new cartridges. If I can find time this winter, the spreadsheet figures will be derived directly from published case drawings and thus easily tweaked to account for thicker brass, deeper bullet base incursions, etc.
The benefit of this from my perspective is the
Ratio column: It' a dimensionless number -- you can start with inches or mm and get the same result.