The "granatepistole", designated HK69A1, is a small 40mm grenade launcher made by Heckler and Koch of Germany. Though it has a retractable stock, it is styled rather like a pistol - in fact, the name means "grenade pistol". Since it is only designed to hold one grenade at a time, which facilitates its small size, it has a "break-open" style of loading similar to some shotguns where the gun is manually opened and the barrel tilts forward for insertion of a round.
The closest it's had to real fame was a cameo in the film Ronin, in which Robert De Niro used it to blow up some cars. It was also featured in an issue of The Punisher's Armory, a semi-educational comic book about weapons and law enforcement/self-defense technology.
It looks like this:
_
___-------------------___________ | \
_ /_________________________________\_______________/__\
| | _--_ |___/ \ \
| | -_ -| ( ) )
| | ||_____\_______________________________________________/__/
| |\____|| | -_
| ||____|| | /
| |/ \| | __________/
| | |_____|__________----------
| | \ / / /
|_| \ /__\ /
/ /_____/
/ /
/ /
/ /
/ /
/_____/