The Speer reloading manual gives a great explanation of how the cartridge gives better ballistics over an open chamber but in a nutshell the cartridge expands when fired sealing the chamber for an instant allowing all gases to go only out the muzzle and pressures can be increased by 4 fold.

For example a .50cal runs just over 10kcup in a sidelock and up to 60kcup in a .308 rifle. I do not know the PSI conversions but they are similiar.