First, the primer explodes. The pyrotechnic primer pellet burns very rapidly, releasing hot gases and particles. These still-reacting components stream through the flash hole and into the combustion chamber and thereby ignite propellant granules that happen to be located near the flash hole. Testing has proven that the primer does not directly ignite propellant granules that are more than about one-half inch forward of the flash hole.
Simultaneously, as granules begin to burn, the entire charge is compressed toward the front of the case, behind case shoulder (if any) and bullet. This compression results from two factors: first, the primer shock wave hammers into the base of the charge and thereby transfer momentum; second, gas generated by inchoate propellant combustion and continuing primer pellet combustion generates gas pressure, which is biased near the flash hole. First, the bullet creates a gas seal at the front of the case. Second, while the charge is initially porous and permeable to gas flow, such flow is retarded because the conduits are tiny and convoluted – with the entire combustion event completing within about 1/500-second; gas pressure simply does not have time to equalize throughout the charge.
Therefore, as burning generates more gas, pressure is progressively biased toward the charge base; hence, the unignited propellant mass progressively compresses, thereby plastically deforming the granules. This is a self-supporting reaction, the greater the compression, the harder it becomes for gas to pass into the charge (conduits become ever tinier and ever more convoluted) so that, very soon, the unignited mass is essentially impervious to further gas