At this instant, two dramatically significant things occur. First, through transfer of momentum from the pyrotechnic jet, the charge column accelerates away from web of case. Second, as noted, some portion of rearmost end of charge is expanding, due to continuing plume reaction and nascent granule combustion within that volume. Since chamber volume is essentially fixed, the only possible result of this expansion within the rearward portion of the charge is acceleration of charge base toward front of case, which results in additional compression of forward portion of charge. As we will discuss farther on in this piece, this compression is not theory but is a well-proven fact, recognized by those who have actually studied such matters.
Obviously, initially, these generating gases will achieve some penetration into the unignited mass but for an entire suite of reasons, this penetration cannot reach the bullet. As these gases move into the propellant mass, several things happen: first, cooling of plume gases; second, compression of gas entrained within propellant column, ahead of plume, which builds interstitial pressure and hence thwarts plume penetration; third, continuing compression of unignited propellant mass, so that eventually permeability disappears, through compressive deformation of granules and subsequent closing of interconnected openings.
Charge compression coincidentally deforms granules, while shrinking interstitial pore spaces between granules. Critical to this discussion is the empirically demonstrated fact that 3000 psi