Originally Posted by
shooternz
The brass flows forward under pressure from 50,000 psi gasses it also pushes the casehead back against the bolt the action flexes and the headspace of the case is longer, the brass being malleable springs back to it's original size as the brass work hardens from repeated firings it loses the ability to return to it's original size and you need to full length size the case, the pressure also pushes brass into the neck making it longer also some people believe that pulling the expanding button back through the neck lengthens it,
once the neck gets too long it will crimp the bullet and cause super high pressures I have seen one rifle blown up because of this,
recommend practice is to trim the case 10 thou under maximum length, After a number of firings the brass gets too hard to spring back
and needs to be annealed or replaced.