From The TimeSplitters Wiki

Minigun
Ammo	400
Fire Rate bullets per sec/cartridge time	High Fire
Reload Time	N/A
Damage (per Single Shot) Head/Body/Leg	?
Type	Automatic
Zoom	None
Primary Fire	Fire
Secondary Fire	Barrel spin
Games	TS, TS2, TSFP

The Minigun is a chaingun. It has an extremely high rate of fire, though if you fire continuously the barrel will overheat. The alternate fire keeps the barrel "warmed up" so you can fire immediately.