Use the gun to shoot endoskeletons, Hunter Killers, orbs and other enemies.
Gradually, the gun overheats, so a constant supply of coolant (found in crates) is required.
Occasionally, extra energy, super shots, super missiles and armour can be found.
Also, killing humans is strictly forbidden, and if any are killed, points are deducted from your score at the end of a level.
This is a very popular gun game even today!