From The TimeSplitters Wiki

Harpoon gun
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Ammo	12/?
Fire Rate bullets per sec/cartridge time	1 harpoon per second
Reload Time	1.5s
Damage (per Single Shot) Head/Body/Leg	?
Type	Heavy Weapon/Other
Zoom	None
Primary Fire	Fires a harpoon
Secondary Fire	Same as primary
Games	TS:FP

The Harpoon gun is a powerful medium range weapon. In Story mode, this is found during the What Lies Below and You take the high road levels.

Tips

The harpoon gun is useful for stationary enemies that are quite far away, particularly if you at different heights. Harpoons can be reused after an enemy has died or they miss, so you can restock the ammo lost so long as they are in a recoverable area. Since harpoons take time to travel through the air, they aren't the best first choice for long-range precision shots and since there's a delay between shots, they aren't good for high-intensity close-quarter combat, but they are good for all the situations in between.