I looked at doing this with small ball bearings at one point, but even spheres have more than one packing configuration they can get into, so a fluid will always be most precise. Bison Armory sells
a pneumatic gadget that makes the measurement based on how much a metered volume of air raises the pressure in the case. However, since it is the as-fired volume of a case that determines its volume's effect on pressure for any cartridge peaking at a pressure that sticks the case to the chamber and stretches on firing (usually bottleneck cartridges that peak at over around 30,000 psi), you would spend less money shooting and then measuring with water before decapping the case.