I just read it. I saw a made for TV movie that explained most of this story a while back, good flic. I think it was called "Supergun" or something like that.
Those super guns are really really big. Even the small one they made was about a hundred feet long and built into exposed bedrock. They can't be moved or hidden. The whole thing takes up huge building complex to operate. A hidden full sized super cannon, once fired, will be a sitting duck.
The Germans also had a love for giant artillery, but as everyone learned, they were largely symbolic and did little damage as compared to conventional delivery systems. They also were all easily located, identified and later destroyed. These super cannons are just a big waste of money, except for maybe the potential for launching low earth orbit satalites. The V2 was much better as was the scud missile.