From The TimeSplitters Wiki

The monkey gun is an extremely powerful gun that fires a full clip of 64 bullets in a few seconds. The reload pace AND looks bear resemblance to the mag-charger. This weapon's only problem is that you run out of bullets VERY fast, even for fully automatic guns. This weapon doesn't have an alternate fire mode and it must be unlocked by completing the electro chimp challenge with a bronze 'trophy' or higher. I don't recommend using this weapon in any team combat modes with friendly fire on.