The Flak Cannon is a staple of the Unreal series and has been in the game since the very beginning. In general, the primary fire launches melted shrapnel, and the secondary fire launches a flak shell bomb. The primary differences between the Flak Cannon's incarnations are the damage it causes, the number of times the flak shards bounce, and the distance the flak bomb flies.
It is the weapon of choice in many situations, like close combat or tight corridors, but can be deadly anywhere if it is used by a master. A Flak Monkey.