I recall reading an article in Blackpowder by Jim Tanner (he is famous for his bullet and ball moulds moulds). He was refurbishing a Remington revolver and described chamfering the chamber mouths. His explanation was that it would cause the ball to swage into the chamber rather than be cut, and that this increases the area of rifling engagement. Don't recall much more detail and it may be that the particular gun had shallow rifling. I make no comment as to the effectiveness but he seemed to think it worthwhile.