The Compound Crossbow is a single shot bolt loaded weapon found in almost every weapon spawn location.
The low audibility rating of the Compound Crossbow makes it great for raiding cities, however its poor rate of fire, lack of ammo storage, and low range make it easy to drop in lieu of a more reliable weapon.
Once fired, bolts will become lodged in whichever surface they impact, and this makes the ammo last a long time. Occasionally the bolt will become irretrievable after killing a zombie, however this can be fixed by using the "Hide Body" action on the corpse. Sometimes, however, hide body will hide the crossbow bolt as well.
To effectively aim the Compound Crossbow, draw a line from both ends of the ironsights towards each other at a perfect 90 degree angle. Where the two lines meet is where your bolt will strike.
While the damage of the crossbow is less than the amount needed to kill a zombie in one hit, it often does so anyway. Whether the damage calcuation on the crossbow is unusual or bugged is not known, but only take that value at it's face. It is possible, that because of the collision detection of the bolt, that the bolt is registered as hitting twice, or that the multipliers of the upper chest and head shots are expanded on purpose due to the nature of the weapon.