If you followed the advice above, earned the Zombie Genocider achievement and then finished the game, congratulations! You've unlocked one of the hottest weapons in the game - the Mega Buster. This Mega Man-inspired firearm is good for 300 shots at a time and will lay waste to anything in its path. Hello, one-shot kills. When you start a new game after unlocking the Mega Buster, you'll find it in the security room behind the air vents. Enjoy!