The Annihilator Pistol will LITERALLY blow enemies to pieces!
Head over to the furnace room on the East side of the map underneath the teleporter. There you'll find the Annihilator Pistol hovering over the flames in all its glory. Pick up the weapon and you'll have completed the Easter Egg! Try out your new weapon on some jealous zombies.
Congratulations on completing the Easter Egg and thank you for choosing this tutorial site to learn from. I hope you learned something new from the tips provided. In fact, you can head over to the 'Hints & Tips' page if you're still struggling with a topic or if you want to improve your zombie killing skills!