caliber, this column will be shorter and contain a smaller percentage of the total charge. Compare sketches 1a and 1c.
The pertinent point is that a primer-generated shock wave travels through the powder column, ahead of the flame propagation front. As noted above, this energetic wave compresses air in interstitial spaces while compressing and deforming granules. Details of this shock wave propagation are of little importance to this analysis. The critical point is what happens as that shock wave reaches the case shoulder, where some portion is reflected. With regard to early ignition of granules in the column behind and adjacent to the bullet base (which are those that can enter the barrel as an unignited mass), location and size of the zone where these reflected waves subsequently focus is critical.
Analysis of pressure wave propagation in cylindrical columns, which relates to a similar analysis By did when he designed a World Record setting 2-Cycle engine exhaust, is not trivial. I will gladly abstain from trying to present anything resembling a rigorous discussion; rather, I will simply report what By calculated.
First, at a shoulder angle near 60-degrees, reflected shock wave energy from the case shoulder is directed back, toward the primer. See sketch 4. Therefore, essentially zero reinforcement of primary shock wave induced heating occurs where it is needed. Equally, at a shoulder angle of about 30-degrees, reflected shock wave energy from the case shoulder is directed toward the central powder column, near the bullet base, where it focuses and generates significant reinforcement of primary shock wave heating, thereby bringing about faster subsequent ignition of this ballistic-consistency-critical mass. See sketch 5.
Shoulder designs with angles shallower than 30-degrees seem to be of little benefit in this regard. Moreover, such designs focus more energy on the bullet base and therefore increase the likelihood that the primer blast will dislodge the bullet, which is detrimental. Angles steeper than 30-degrees seem to be progressively less beneficial, owing chiefly to lack of proper focusing but also owing to an increase in magnitude of axial shock delivered to the barrel through the chamber shoulder, which is also detrimental.
Shock Wave Velocity in a Powder
Column
The primer blast generates a sonic shock wave. This wave moves
through the powder column in a complicated manner – depending upon
column diameter, column air density, powder granule solid density
& elasticity and almost certainly the manner in which granules
interact with each other, interstitial air and the case walls. (Shock
wave velocity has been precisely measured in specific applications;
however, that information is hard to obtain.)
We can, however, make certain reasonable assumptions and predict a velocity of about 1800 fps. This suggests that in the 308 Winchester, shock wave arrival at the case shoulder occurs about 0.069 milliseconds after production from the flash hole. The reflected shock wave focuses at the center of the powder column behind the bullet by about 0.072 milliseconds – long before generated propellant gas pressure is sufficient to move the bullet.
As this primer-blast shock wave travels through the powder column, both air and powder granules are compressed. As this pressure wave passes, granules are fluidized and the entire affected volume can obtain a consistency similar to bread dough. Typically, as this wave passes, developing propellant gas pressure, which travels through the compressed column faster than the initial shock wave travels through the undisturbed column, immediately reinforces the pressure, thereby maintaining granule fluidization.