The book was "Day of the Jackal". A professional assassin was trying to take out the President of France, Charles DeGaulle. The stated theory was that the heavy mercury, upon rapid decelleration, would slam forward, blowing the bullet apart.
Total pure fantasy. The author confused mercury, a heavy toxic metal, with fulminate of mercury- a REALLY touchy explosive that USED to be used to make primers, blasting caps, etc.