(click photo for source)
It was build to reduce noise to communities nearby. The vehicle pictured is the M109G 155mm self-propelled howitzer.
There was some debate as to whether these photos were real. After some dismissal, the military guys started writing on the subject, here is one poster:
I actually saw one of these. It is a silencer (muffler) and it is German -- you can (sort of) make out the black cross on the side of the howitzer outlined in white, plus they do use that cammo.
This is a static firing range -- allows the Howitzer to live fire, and allows enough movement of the barrel that they can hit or miss the target, so they can see how accurate the crew is... But at the same time, muffles the sound, because like all of Germany, there is a town or village nearby that they don't want to disturb with the constant sound of live fire heavy artillery practice.