Credit to VirusisPro for uploading the vid.

Text Tutorial:
1) Create a class with any assault rifle with the Flamethrower attachment.
2) Choose any camo on the assault rifle. (THIS WILL BE ON THE CROSSBOW)
3) For secondary add the crossbow.
4) Spawn with the class.
5) Switch to the Crossbow, then press D-PAD LEFT to pull out flamethrower.
6) Throw a semtex/grenade to kill yourself.
7) While the semtex/grenade is going off switch to another class that DOESN'T have the crossbow.
8) Walk to the spot that you died and pick up the crossbow.
9) Enjoy your super-badass crossbow!