Real Mega Buster

From HelpMeWithMyGameProblems.com

This powerful weapon can be used to kill most of the psycopaths in the game in between one and five shots. The weapon can be found in the Security Room after the Zombie Genocider achievement has been awarded, and the game in which that award took place is completed. (I.e., the full 72 hours of game time are allowed to elapse.)