A Hard sound rifle is a human-made sonic weapon originally intended for use in assassination missions against political leaders. As it utilizes concentrated sound waves instead of a solid projectile (such as a bullet), a Hard sound rifle kills the target instantly and leaves no residue or powder burns and causes little noticeable external damage to the intended target, making the weapon virtually untraceable in forensic investigations.
This weapon usually causes damage by obliterating internal organs. It is a preferred weapon by assassins of the Office of Naval Intelligence, though they are occasionally illegally acquired by civilians, oftentimes for less malevolent means. The Hard sound rifle can be used as a more efficient and more stealthy variant of a Sniper Rifle.
Although sonic weaponry doesn't appear in any Halo games, it is mentioned prominently in I Love Bees when an ONI assassin, disguised as a homeless man, was sent to kill Rani Sobeck for her investigation into a covert evacuation of the Human colony of Troy, a protective detail dispatched by Colonel Herzog eliminated the operative through use of a hard-sound rifle. Having fallen off the roof to the street below, local authorities dismissed the death as an accident resulting from use of alcohol or drugs.