MuskegMan,
The primer doesn't "feel" anything. Instead, the flash-hole is just the first "escape channel" for hot powder gas, just as the lowest point in river levee is the first for high water.
Once again, the reason primers back out very early in the build-up of gas-pressure is because they're far lighter and less "tight" than the bullet. I already gave you a very graphic example with .30-30 ammo--but a less obvious example is MORE primer flattening in new brass in higher-pressure cartridges. That occurs because there's often a little more headspace with new brass, again because the primer backs out before the case starts to expand. If the same load is fired in once-fired, neck-sized brass, the primer won't be nearly as flat.
