I'm a technician. That makes me a professional picker of nits because if I don't get it right the first time, 200 people will suffer a fiery death at 30,000 feet.
TWR, if it were pressure alone, why does a 300 Winchester Magnum need a larger capacity case (combustion chamber) to make more velocity than a 308 Winchester? Why not just increase the powder charge (and thus, pressure) inside the smaller 308 case until 300 WM velocities are achieved? Because it leads to dangerous pressures. For higher velocities, more powder is needed, but the more powder used means case capacity must be increased to keep pressures at a safe level. For the same pressure, the larger capacity of the 300 WM produces more velocity.
Increasing case capacity is just one way to increasing the volume of the combustion chamber. Another is to use longer leade. When the primer ignites, it quickly builds up enough pressure to push the bullet into the rifling which will stop, or, at least slow slow down the bullet's forward motion. That's about where the pressure spikes. This creates additional combustion chamber volume letting the extra powder burn within safe pressure limits. It's a trick Roy Weatherby used back in the fifties to get more oomph out of his cartridges
Last edited by MistWolf; 05/27/16.
