What is the case capacity and why does it matter?

Case capacity refers to the internal volume of a cartridge case – essentially, how much space is inside the brass for powder (and combustion gases) and the seated bullet. I

n reloading, case capacity is often measured in terms of how many grains of water the case can hold (with the spent primer in place) up to the brim. For example, a .308 Winchester case might hold on the order of 56–58 grains of water, whereas a much larger case like a .338 Lapua Magnum can hold over 114 grains of water. 

The exact capacity will vary by brand and the lot of brass. It’s important to understand that case capacity is essentially the available combustion chamber inside the cartridge. When the cartridge is fired, the burning powder generates hot gases that rapidly expand. The volume those gases have to expand into (the case’s internal volume plus the barrel space behind the bullet) will influence the pressure that builds up. This is why even identical caliber cases can behave differently if their internal volumes differ.

• See here how you can measure case capacity.