From The TimeSplitters Wiki

The Sci-Fi handgun is easily the best pistol in the game. It can fire as quickly as you can pull the triger, has a reasonably large clip, is very accurate and has a high damage rating, and also a small zoom ability. This version is a vast improvement on the Sci-Fi pistol of TimeSplitters 2, which was generaly recived as a very poor weapon. This version has the ability to fire ordinary blue plasma round, which are high damaging, or to fire the traditional 'rebounding' rounds, which are green and slightly less powerfull. As always, care is advised whilst the rebound function is active, as shots fired at a wall right infront of you have an alarming tendancy to bounce back at you. The compusary 3-shot-burst has also been removed from this version, makeing it a much more user friendly weapon. It has a magazine capacity of 16 and a large amount of reserve ammo, making it a very usefull weapon in your arsenel.

Of course, two Sci-Fi handguns are better then one.