Shear Versus Compressive Strength
One more point, some
have identified the unignited heterogeneous mass as a potential bore
obstruction. Such a deduction evidently stems from ignorance about
characteristics of such materials. Just as with sand, granular materials
can exhibit tremendous compressive strength, while having very little
shear strength. Since the unignited material can shear at relatively
low stress, it cannot represent a significant bore obstruction – in
a significantly bottlenecked case, the center of the unignited mass
simply shears free from the perimeter (which is trapped behind the
case shoulder) and pushes the bullet down the bore. (Granules can
either slide past one another or shear through at a relatively modest
pressure.)
Conclusions
Reviewing the above discussions supports the
conclusion that the primer plume never enetrates full-length of the
charge in a typical rifle-type cartridge. Conversely, in most (shorter)
pistol and revolver cartridges, the primer plume usually does reach
the bullet base unless unusually dense charges of relatively slow
burning propellants are used and even that influence has a limit near
½-inch of propellant column length.
Without significant evidence from direct testing, attempting to place absolute values on effective depth of primer ignition penetration would be foolish. Nevertheless, we can say from experience, anecdotal information, QuickLOAD, SMc™ results and limited laboratory data that it probably commonly exceeds ½-inch and seldom significantly exceeds ¾-inch.