From The TimeSplitters Wiki

A futuristic assault weapon that has an increasingly high rate of fire until it overheats. It then needs to vent for a few moments to cool down. Secondary fire shoots sticky plasma grenades.

Plasma Autorifle
Ammo	200 energy cells/20 plasma grenades
Fire Rate bullets per sec/cartridge time	N/A
Reload Time	N/A
Damage (per Single Shot) Head/Body/Leg	N/A
Type	High Fire Rate/Grenade Launcher
Zoom	Yes
Primary Fire	Fires energy cells
Secondary Fire	Fires plasma grenades
Games	TS2

The ultimate fighting weapon of the future, made of solid titanium, it can unleash rapid bursts of plasma as well as firing time-delayed grenades. Be careful not to let the rifle overheat, as the cool-down period can give enemies time to deal some serious, perhaps even fatal, damage.

Of note is that the Plasma Autorifle's power seems to have been toned down in Arcade and other multiplayer game modes, somewhat reducing its usefulness when compared to Story Mode.