Special Ammo from a regular big ole sniper rifle. Smart, Silent, Armor Piercing, Explosive, and untraceable as the bullet fragments all somehow disappear after being fired. Some kind of chemical (or alchemical) round that starts out solid, has amazing flight characteristics, an exothermic (i.e. explosive) reaction when it comes into contact with water (i.e. people), and leaves no traces after the reaction (i.e. the explosion).
Very expensive to manufacture, 100,000 Nuyen a round or some such. Should do the trick.