Unclenick mentions:
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Actual case volume has to be measured because outside dimensions are often not exactly the same.
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I've never been a fan of determining case volume by measuring the weight of water that just fills the case. The only real case volume there is that counts is when the case is pressed hard against the chamber walls and bolt face. That's when all cases have the exact same outside dimensions and only their inside capacity is the space the powder gasses are at. Thicker walled ones will have less volume for a given chamber and usually more weight as all cartridge brass is pretty much the same metalurgy. So I think case weight is a better measure of case volume; thinner ones will weigh less and therefore have more room inside when they're fired.
One could calculate the chamber volume (bolt face to chamber mouth with high school math then with the weight of cartridge brass calculate how much brass in grains it would take to fill the chamber. Then subtract case weight. The difference is the case volume when the round's fired. Cartridge brass (70% brass, 30% zinc) weighs about .288 pounds per cubic inch.