Primary: Press the or button and Chuck / Frank fires a blast of high-frequency sound waves that will burst the heads of gas zombies, killing them. It will knock down regular zombies without doing damage, and cause most survivors and a few psychopaths to bend over and vomit violently for a few seconds.
Using it on Mark and Pearce will cause them to throw up, allowing you to get up close for an easy kill. This vomiting effect applies to almost any survivor you shoot it at.
This will NOT kill or even damage regular zombies, only knock them down. However, you can always use this if you get in a jam and need to make you way out to heal yourself.
Though it doesn't hurt regular zombies, shooting them with the BFG will still count toward He Hasn't Covered Wars...
In the Secret Labs after the failed military rescue, after defeating Mark Bradson and Pearce Stephens. It is beside the lever Chuck pulls in the following cutscene.
The abbreviation "B.F.G." is a reference to the Doom Series, however, unlike in Dead Rising 2 in which the acronym stands for Blast Frequency Gun, the Doom acronym originally meant "Big Fucking Gun" or "Big Freaking Gun" (according to the Doom motion picture the acronym stands for "Bio Force Gun").