Over chamfering will ehlp you end up with shorter cases as the bevel will only go so far the you run out of neck wall and the case will get shorter. The reason for chamfering the necks is to keep the bullet from being shaved away by the sharp lip on the inside of the case.
when you chamfer the insides of my cases I cut just enough to make a shiney brass ring , that way you haven't taken away from the gripping surface of the neck and you have nocked the sharp lip off of the inside of the case mouth.
As for the outside chamfering I have yet to see a need for it.
