testing with guns equipped with glass windows in the barrel. They then use high-speed cameras to take photographs at increments along the bullet path.
Such photographs, taken near the chamber, always show a plug of propellant pushing bullet through bore – of course this plug is burning along its rearward face and may burn out long before bullet reaches muzzle.
Stress-versus-strain properties of propellant granules are an important consideration in this discussion. Testing proves that when subjected to extremely rapid loading (stress), as occurs with the shock of a primer blast coming through a flash hole, propellant granules react as an extremely rigid solid (resistant to deformation) and an extraordinarily tough material (resistant to shattering). At lower loading rates (as with the buildup of pressure during nascent granule combustion), propellant granules deform more easily but are still quite resistant to shattering.
Additional support for the contention that the primer blast does not penetrate significantly into propellant column and certainly does not simply blast a hole through that column comes form a simple test. Filling a primed 44 Magnum case with W296, which is notoriously difficult to ignite, and firing the primer results in unignited W296 spraying from muzzle with very little, if any, propellant left in case, chamber or barrel. If the primer blast simply bored a hole through the propellant column, this test should leave a significant portion of propellant in case, cylinder and barrel.