Very little is known about the Assault Cannon. You have first-hand knowledge that it was developed by the Empire for use with the Dark Trooper project. The cannon is so large that most people probably couldn't even lift it. The weapon fires bursts of plasma at a rapid rate(~4.2 shots/s) as it's primary capability. The secondary mode launches a long range incendiary rocket. The plasma is relatively accurate, but the missiles are best used for larger targets.
This weapon packs the biggest punch of anything you'll come up against. The blue plasma charges will take out any Imperial with one blast while some of the tougher species will require two or three. The missiles are very good for taking out large groups, just like any explosive weapon. But the assault cannon is best used for long range attacks against Dark Troopers. The plasma bursts won't do you any damage in close quarters but the missile explosions will. Use with care.