It all boils down to (pun intended) thermodynamics. Boyle's law applies, but our systems are dynamic in several ways such as cartridge/bore cross-sectional ratios, bullet moving, bullet accelerating, rate of deterrent being burned off the surface of the powder kernels, shot initiation pressure (system begins as a closed one and then energy is dumped as the bullet's static inertia and engraving resistance is overcome), and orobably some I missed. It's complicated but not beyond understanding. Quickload operates on basic thermodynamic formulas, specific heat, and physics 101.