infiltration. Thereafter, no significant amount of additional hot gases can infiltrate; subsequently, ignition and combustion can only occur at exposed surfaces. Even while gas can penetrate, because the front of the case is essentially perfectly sealed by the bullet – no bullet canbullet movement has yet occurred when this all happens – insufficient heat can be carried into the charge to result in ignition beyond the first quarter-inch, or less, of the mass that was not directly ignited by the primer.
Cylindrical versus Bottlenecked
Compare the 308 Winchester and the 45-70 Springfield. In each, the primer will ignite about one-third of the total charge. Before compression can seal off the rest of the charge, secondary ignition into the rearward surface of the unignited mass will penetrate sufficiently so that, perhaps, another one-sixth of the initial total will ignite. Hence, about one-half of the charge will be ignited; the remainder will be a more-or-less solid chunk that is burning only on the rearward face.
As chamber pressure continues to build, pressure acting through the unignited propellant chunk eventually becomes sufficient to begin to force the bullet into the bore. To do so, in the 45-70, all that is required is that total force on bullet base (force on base of chunk minus friction between chunk and case walls) exceeds force required to push bullet out of case.