If you can find the Repeater X crossbow in World War Z, then consider yourself lucky.
The Repeater is very accurate, and the bolts explode with a slight delay after hitting their targets. The range on this weapon is also very generous, so you don't have to be close to zombies to kill them efficiently, giving you room to plan your escape or set up your defenses.
To make the most of the Repeater, use it on a swarm. Explosions from its explosive bolts will deal splash damage to nearby zombies. This will save you ammo in the long run.