"Day of the Jackal", book about a sniper assasin ~1974. The guy drilled out a small amount of lead from the bullet tip and injected liquid mercury, then sealed it. This causes the bullet tip to "explode" upon impact. Similar to a soft point bullet but far more damaging to the intended target. More energy is transferred to the object that is hit because it immediately mushrooms and splinters shredding more tissue than a regular bullet.