imgoofy,
The easiest way to accurately measure water capacity is with a fired case, for two reasons. They not only more accurately reflect the capacity of that chamber, but the expanded neck allows a bullet to slide easily inside--and allow excess water to escape. (If a bullet won't slide easily into a fired case's neck, then the neck is too thick for that chamber, and needs to be inside-reamed or outside-turned.)
Fill the case with water to the mouth, then stand it up on a flat surface and push a bullet into the mouth until it's at the appropriate overall length. (I often use a ruler marked with the loaded length, but also often use a cannelured bullet seated to the cannelure.) After the bullet pushes out the excess water, I pull out the bullet and wipe any droplets on it into the case mouth, then also use a paper towel wipe any water from the outside of the case. Unless the case is turned upside-down, surface tension will keep the water inside, allowing it to be weighed.
I ran some comparative tests in my NULA .257 Weatherby Magnum with IMR7977 and H1000 when the first Enduron powders appeared. In that rifle, with those lots of powder, velocities were within 20-25 fps of each other with the same bullets and powder charges.
