The Eightball Gun cannot, in fact, launch eight rockets at once, but it can launch eight waves of six rockets / grenades. It has two modes: rockets and grenades.
The first mode launches slow moving rockets that have different modes and flight paths.
The first firing mode is simply standard firing of rockets while the second one is a heat seeking mode. To have this mode enabled, you have to aim your target for two seconds without doing anything with the weapon, and soon the crosshair will become a red circle and transform the rockets into guided missiles.
For the different flight paths, there is a 'Squadron' version and a 'Circle' version. For the Squadron flight path, the rockets will fly in a straight line if you use the regular fire. The Circle flight path comes into play when, after six rockets are loaded, the alt-fire is pressed, leading the rockets to shoot in a tightly packed circle.
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Finally, the Grenade mode uses the secondary fire mode. It is essentially a grenade launcher, shooting bouncing grenades that will explode on contact with almost anything - be it a player, a monster or even a dead corpse.