armyrat:
What happens is that the recoil of the firearm actually pulls the brass off of the bullet.
For example: Since the brass of a .357 Magnum case is somewhat lighter than the bullet, it's easier for the brass to be moved backwards during the recoil than it is for the bullet. Yes the bullet is for a fact subjected to the exact same recoil, but inertia comes into play here.
Think of a stalled car... When someone is trying to move that car by hand you know that it takes much more effort to start the car moving than it does to keep the car moving.
Same thing with the casing of the cartridge versus the bullet, it's easier to start the case moving than it is to get the bullet to start moving.
Jim