From The TimeSplitters Wiki

The crossbow is a weapon from Timesplitters 2. It appears in the 1920 Aztec jungle level, as well as some challenges and arcade league levels. It has weak attack power, but you can light a clip on fire to increase the attack power. This weapon is replaced by the Harpoon in TimeSplitters Future Perfect. You can also pick up the bows left on the ground if you miss your target.