It is worth noting that quenching of granules that happen to be adjacent to the case walls or to the bullet base can also render impinging primer gases incompetent toward ignition. This is one reason we find unignited granules expelled from combinations such as a 38 Special, firing a load using 2.7 grains of Bullseye, which ignites quite easily, despite ignition by a primer that easily touches off a load using more than 20 grains of H110, which is very difficult to ignite. While we cover a mitigation of this problem in another aspect of our patent, said aspect is of little interest to most modern target- or hunting-rifle shooters so we will ignore it herein. However, those aspects of our patent can significantly benefit shooters using typical blackpowder and handgun cartridges; we anticipate covering this information in a future article.
As nascent combustion of ignited granules creates gases that increase pressure within the combustion chamber, granules and entrained gases within the propellant mass continue to deform. Through this process, propellant mass volume decreases by perhaps 15%. For complicated reasons (including compressibility factors), little additional compression occurs beyond that point – as applied pressure exceeds about 3000 psi, the relatively incompressible granules deform significantly, resulting compression of entrained gases heats and pressurizes those until gas density becomes sufficient that ideal gas laws no longer apply.
Compression of gases within trapped inter-granule pockets does result in significant adiabatic heating of those gases but those pockets are generally far too small to contain sufficient heat energy to provide granule ignition through this means – conversely, in rocket motors, larger gas pockets can and do result in such point ignitions, thus so many early explosive failures in solid-fuel rocketry! Similarly, this effect may contribute to "detonations" when nimrods use relatively reduced charges of slow burning powders in large cartridges.
Depending upon many variables, as chamber pressure reaches perhaps 3000 psi, the bullet begins to move into the barrel. At this point, one of several things can happen. Generally, in a typical bottlenecked case, a shear zone forms through the propellant mass and thereby an essentially impermeable, bore-diameter plug of unignited granules pushes the bullet into the bore while a cylindrical mass remains trapped behind the case shoulder. (One could say that the plug merely follows the bullet into the bore but that is not quite accurate; since this plug is largely impermeable, it must transfer some portion of the combustion force to the bullet.)
Vagaries of shoulder design significantly affect what
happens to the cylindrical portion of the propellant mass. Any of
the following things can happen to some or all of this mass: squeeze
down and extrude into bore; tear asunder – as granules exposed at
the interior surface are ripped free by turbulence generated in rapidly
passing gases; remain trapped behind case shoulder. In any instance,
this shear zone (created as a bullet-diameter plug of powder pushes
the bullet into the bore) creates a substantial increase in ignited
surface area. The burning front at the exposed surface of the propellant
mass thereby extends forward along the surfaces of this shear zone.
Thereafter, propellant within four distinct burning regimes exist:
1.
individual granules that had been ignited by the primer and continue
to burn;