The BFG 9000 does not behave in a simple poing-and-shoot matter. It has a characteristic called "Blast Field". This is like a cone that spreads out from the marine. As you shoot the BFG, the cone fans out from you in the direction in which you shot the blast.
If you shot North, the cone will travel to the North of you even if you turn South or another direction. It executes at the moment of the detonation of the green ball. This blast is very powerfull.
Example: say you are at the hallway much similar to the Map01 hallway. You shoot the green ball down one hall, and so, the blast field will travel with you down that direction at all times. Your victim rests at the end of the other hallway. Therefore, you may run to the other hall and when the green ball detonates, your victim will turn into hot cake. :)
My own little drawing to show the weapon's operation. :)