Under some circumstances, these pockets might be large enough and become hot enough to heat adjacent granule surfaces sufficiently to cause ignition. This potentiality is particularly noteworthy in those combinations where the case is not filled with powder. There, granule heating through primer-blast-related compressive forces can be significant. This is almost certainly a factor in reduced-charge "detonations" with slow powders. Indirect and lesser effects of this phenomenon could also explain why the most accurate loads seldom use significantly less than a case-full charge.
"Kindling" Versus Ignition
Here
I must clarify a significant detail. Some readers may not recognize
the important distinction between kindling point and ignition point.
For this discussion, we can consider kindling point as that temperature
where smokeless powder ignites in response to very slow heating –
temperature gradient in near surface layers is very modest. Conversely,ignition point is that temperature where smokeless powder ignites
in response to very rapid heating – temperature gradient in near surface
layers is significant. This distinction is critical, particularly
when discussing late stage ignition – where granules escaped ignition
either from the primer blast or through the action of developing propellant
gases until after the bullet began to accelerate.
Kindling point is also known as Thermal Decomposition Point. This is defined as that temperature where more heat is generated by decomposing propellant surface layers than is conducted into the propellant (exothermal reaction). Subsequently, in order to maintain granule surface temperature, it becomes necessary to remove heat.
Ignition Point depends upon heating rate because the faster that heating occurs, the hotter the surface layer can get before the temperature gradient in the surface layers exceeds the rate of thermal conductivity between those layers. Consider heating to any specific surface temperature: when heating occurs slowly, the heated zone gets relatively thick, so that underlying cooler layers cannot rapidly wick away additional heat; when heating occurs rapidly, the heated zone stays relatively thin, so that underlying cooler layers can rapidly wick away additional heat.
Kindling (thermal decomposition) point for various typical smokeless powders is listed as a surprisingly mild 160-170 degrees C (320-338 degrees F). This reflects the temperature of slowly heated surrounding air when a sample spontaneously ignites (exothermal reaction ensues) during very slow laboratory heating. Such ignition is completely unlike what happens in a cartridge. There, condensation of primer gases onto granule surfaces results in extremely rapid heating. In this instance, if we could measure granule surface temperature at the instant of ignition, we would find that it was far higher than the laboratory-derived thermal decomposition point. A demonstration that ignition point depends upon heating rate.
This analysis applies to those granules not ignited by the primer. While the various forces involved in the ignition phase and during nascent propellant combustion typically generate granule surface temperatures far exceeding the thermal decomposition point (up to 2650 degrees C – 4800 degrees F), such granule surface heating occurs extremely rapidly; therefore, ignition temperature is a function of ignition point, not kindling point. Furthermore, once powder gases generate sufficient force to begin to move the bullet, any unignited granule typically has far less than 2 milliseconds (2/1000 seconds) to achieve combustion before the bullet reaches the muzzle. The bottom line, it is quite common for such granules to escape ignition….