I think one of the best wacky weapons of WW2 is the bouncing bomb:

Actual Bouncing Bomb Movie

I've seen something similar and, perhaps, a little whackier that was used by the British Home Guard at a time when there was a real fear of invasion, and much of the British Army's equipment had been lost at Dunkirk.

A semi cylinder with two large wheels, the tow bar was a gun barrel. After being wheeled into place the device was flipped on it's side so that it traversed on the wheel it was sitting on. Potentially more dangerous to the user than the target, desperate measures for desperate times. It seems the comic antics of "Dad's Army" are a little more real and more serious than I thought.

http://www.ww2incolor.com/homefront/SmithGun.html

Portable bunkers like this were intended to quickly supplement static defences with hard points and would most likely have been dugin so only a small part would be visible, and have mostly been around since at least the 19th century so considerably before WW2, this version is just another spin on the concept originally I believe started by Gruson in the 1880s...

http://www.lonesentry.com/articles/mobilepillbox/index.html

Colt .45 pistol as a fully automatic version. Some were carried by aircrew in the USAAF in WWII.

http://www.thefirearmblog.com/blog/2010/03/29/colt-m1911-machine-pistols/

Mike Bunkermeister Creek
Bunker Talk blog