Comments so far:
MartinCreek,
In general, Federal primers often do flatten more than CCI, Remington and Winchester primers, because their cups can be thinner. In fact, I've owned both a rifle and a revolver that would go bang consistently with Federal primers, because the cups were more easily dented by the firing pin. It turned out the mainspring on the rifle was going bad, and eventually it wouldn't make any primer go bang. The revolver was a Ruger Redhawk with a "spring kit" installed, which made the trigger pull a lot better but also made Federal primers necessary.
Centershot,
Protruding primers just might contribute to flattening! (Now I might have to run some experiments.)
Muskegman,
Yes, the "pressure inside the case is equal in all directions." But that is true of the eventual maximum pressure. Initially, pressure always finds the path of least resistance.
In this instance that's through the flash-hole, rather than at the other end of the case where the bullet starts moving. Even in very small calibers, the bullet is not only heavier than the primer, but usually held far tighter by the case neck, because of more contact with the neck and, sometimes, crimping. (It takes a lot more force to seat a bullet than a primer.)
Plus, the normal "high" primer of fired .30-30 cases indicates the primer was backed out the initially relatively low pressure.
