Before leaving this section, it is critical to understand that the above facts indirectly support many points discussed subsequently in this article. Specifically, if primer plume particles do not effectively penetrate as far as primer plume gases (which is well demonstrated), then it follows that propellant column permeability and porosity are key to the ability of a primer blast to effectively penetrate into said mass.
What Really Happens when A Primer Plume Blasts Into A Propellant
Charge
Propellant granules do not ignite instantly. Compared to generation
of the primer plume from the flash hole, and the subsequent continuing
reaction of plume material within the powder chamber, granule ignition
delay, while not an eternity, is rather significant.
The smallest propellant granule is many orders of magnitude more massive than is any constituent of the plume. Hence, owing to initial permeability and porosity of propellant column, a significant volume of plume material will penetrate into base of said column before the plume jet transfers a significant amount of momentum to granules near the column base.
After entering propellant mass, these reacting gases will spread to fill a roughly spherical, or perhaps a roughly hemispherically ended cylindrical, volume of propellant. Subsequently, expansion of gases within that volume (due to decompression and continued reaction, which increases plume volume, and due to nascent propellant granule combustion) will tend to expand that impregnated volume at the expense of unignited mass volume.