If you can't get a real BFG, you can always resort to creating one out of a mountain of LEGO bricks.
If you played the original Doom, you know all about the BFG 9000. The massive weapon fired huge balls of green plasma, and was easily the most powerful weapon in the game. If you've ever wanted your very own BFG, then YouTube user ZaziNombies has something you need to see:
A BFG 9000 made of about 5,000 LEGO bricks.
The massive creation weighs in at over 20 pounds, and in the video, you can see the scale of the weapon compared to LEGO minifigures.
It's a great replica, and one of the coolest LEGO creations I've seen in a while. ZaziNombies is looking for suggestions for the next LEGO Creation, so if you have ideas, you should definitely pass them along.