Fair enough; basically you want to define it in terms of the thermal efficiency of the conversion of the chemical energy in the powder into kinetic energy of the bullet. To do so, you'll need to specify as well peak pressure, barrel length, and bullet SD.

Since barrel length is to be part of the equation, then your definition will pretty much boil down to expansion ratio. Here, shorter barrels will demand smaller cartridges as will smaller bores.

Last edited by 2525; 04/28/10. Reason: explain