compression squeezes interstitial pore space so that both porosity (air space) and permeability (pathways through which gas can move) are reduced, the latter soon effectively disappears – in the subsequent 0.001 second before bullet reaches muzzle, if gas cannot move through mass, it is impossible for sufficient heat to transfer into mass to ignite granules trapped therein. With granule sizes used in small arms propellants, compressive heating of trapped gases cannot generate sufficient local heating to cause adiabatic granule ignition – however, in some instances of "detonation" this effect may well play a part!
Second, since front of charge is effectively sealed (while some gas can escape past bullet, this effect is modest); due to boundary layer considerations and the almost vanishingly short time available, even while permeability still exists, very little heat energy can pass through unignited mass. Granules that are well removed from zone of primer induced ignition, cannot achieve ignition temperature through indirect gas heating.
All subsequent granule ignition occurs after bullet begins to move and results from various sources, depending upon case configuration and a few other variables. These sources include combustion along rear face of initially unignited mass; combustion along any sheared face that may occur in initially unignited charge (such as occurs in many bottlenecked cases, when perimeter of unignited mass is trapped behind case shoulder while core is driven forward behind accelerating bullet); "turbulent" ignition, where granules are torn free from plug or trapped material by gas flow or friction with case walls, bore interior; and possibly other causes.