The dynamic process that occurs after the primer begins to burn is very complicated. In typical rifle cartridges with typical loads, primer blast results in direct (primary) ignition of a significant portion of charge but certainly not 100% of charge – a significant portion of granules are not ignited by the primer. Those granules not ignited by primer are concentrated front of case.
Interaction between plume and granules near base of charge work to transfer much plume momentum to propellant column. This factor, along with nascent combustion of granules in that zone, works to accelerate base of column, thereby compressing forward portion of charge.
Compression of the granule mass within this closed system retards infiltration of hot gases from nascent granule combustion so that front of charge cannot ignite, except at rearward surface (individual granules, while still hidden within this mass, do not ignite).
Case shoulder can act to disrupt this heterogeneous mass, thereby exposing additional granules and entire surface areas to ignition sources. In a cylindrical case this disruption does not occur.
A Note
about the Sketches
Elsewhere on this web site we have presented sketches
as simplifications of what happens in typical instances of conventional
priming, when applied to loads using case-filling charges of relatively
dense propellants.