Nick Martinelli spent 6 months building a functioning Wunderwaffe DG-2 from Call of Duty. Well, "functioning". It doesn't behave like a Wunderwaffe DG-2 actually does because then he'd probably be in jail, but it does have a built in flame thrower and taser. It's also one of the best looking props I've ever seen so it's only a minor complaint that it can't actually electrocute/murder groups of zombies at a time.
Keep going for the full video of the build.
And what the Wunderwaffe DG-2 is actually like in the game: