Case Volume in gr. of water

[Balljoint]

Came across a Velocity Calculator on a web site i subscribe too http://ammoguide.com/cgi-bin/ai.cgi?sn=ffAxqCLOYD&login=balljoint&fr=&loginreferer=&curr=ai, one factor needed is the case volume in grains of water in a 308 rifle case all other factors looks like i have them.
can anyone one help me

[drybean]

weigh a empty case,then weigh that case full of water
the diff. will be volume in grns. of water
drybean

[Balljoint]

Looked on several web sites and found finally that 308 Mil cases hold 56 gr of water (general) and Factory 308 cases hold 58 gr. water (general) close enough