A Critical
Point about How Primers React
Many readers may have an impression of
the primer as a detonating explosive – "once the striker hits the
cup, the pellet explodes and thereby generates hot gases and incandescent
particles." While accurate, in as far as it goes, this is far from
a complete and accurate picture. What the primer pellet actually does
is burn extremely rapidly (technically a deflagration). This deflagration
does, in fact, generate a stream of hot gases and fine particles but
within this stream is a significant admixture of material that is
actively reacting, long after the plume passes into the propellant
chamber. This latter characteristic is critical to understanding how
a primer actually works in a sporting cartridge. This continuing reaction
results from two sources: first, pieces of unreacted primer pellet
entrained within plume; second, partially reacted molecules that continue
to react toward a more stable (and more energetic) reaction end product,
thereby generating lighter, hotter molecules.
Salient to this discussion is the fact that volume of plume material produced through the flash hole is only a tiny percentage of final volume of plume material generated! This is a significant reason why the generated plume tends to compress the propellant column, rather than blast a hole through it.
The included photograph of a thoroughly typical unconfined primer blast (produced into the atmosphere through the flash hole of a case head, case body removed) irrefutably demonstrates