Case Capacity
Also: Water Capacity
The internal volume of a cartridge case, usually measured in grains of water, that sets how much powder it can hold and its velocity ceiling.
Case capacity is the usable space inside a fired or sized case, expressed in grains of water because water fills the volume completely and weighs in convenient units. More capacity means room for a larger powder charge, which generally raises the velocity a cartridge can reach before pressure limits stop you. This single measurement explains much of why one round outruns another of the same caliber.
Capacity varies between brands and lots of brass because wall thickness and internal taper differ, so handloaders weigh and sort cases when chasing consistency. A case whose capacity is large relative to its bore is called overbore, a design that adds velocity but shortens barrel life. Measuring water capacity also helps you predict pressure when no published data exists for a particular case-and-bullet pairing, since volume drives the burn from the case head forward.