M.L. McPherson, December 2002
How Far Does a Primer Plume Penetrate?
Synopsis: The primer blast does not ignite all of the granules in a typical rifle load. A fundamental characteristic, and ballistic limitation, of conventionally primed sporting cartridges is penetration depth of the primer plume (pyrotechnic jet produced through flash hole). Depth of effective penetration depends upon many factors and is certainly somewhat variable; however, in typical rifle loads, this variability may be surprisingly small. Conversely, several lines of evidence demonstrate that, regardless of cartridge configuration or propellant type, direct ignition of the majority of propellant granules in any zone cannot occur more than perhaps about ¾-inch forward of case web interior surface and that under most circumstances such ignition depth will exceed about ½-inch.
The following discussion may seem to ramble around the issue of what the primer blast does to the propellant column, rather than focusing directly upon how far the primer blast penetrates that column. This is because an accurate understanding of precisely what the primer blast does and how that affects the propellant column is critical to understanding why a primer blast does not directly ignite every granule in the charge.