From The TimeSplitters Wiki

The flare gun is a very powerful weapon in the hands of a skillfull player. For those less practaced, it is ill advised unless used as a last ditch effort. The flair gun has a very high damage ability, similar to that of a rocket, (and can kill the player too at close range) but are only one shot per clip and a long reload time, and the maximum ammo that you can carry is limited to 10 aditional shots. The flairs are inacurate, as the projectile travels in a spiral, and move slowly enough to be dodged at any range. However a direct hit is usualy enough to kill a player outright. Occasionaly they set a player's corpse on fire too.

As with all weapons with the ability, two is better then one, as you can fire the other as the first is reloading.