Its been several decades since I read "The Day of the Jackal", by Frederick Forsyth, but I remember the assassin drilled a hollow cavity deep in the nose of a bullet and put some Mercury in the hole. Then the tip was sealed, perhaps with lead. The idea was that the heavy, liquid Mercury would move forward in the cavity when the bullet hit and slowed down. This was supposed to cause the bullet to fragment in the targets head. He tested one of his bullets on a melon, as I recall, and was happy with the way it "exploded".

rick