Let's start with a pretty reasonable one.
A gun from your biggest mass produced heavy tank in a medium tank turret? Easy. Let's kick it up a notch.
Players of World of Tanks will already be familiar with this cannon. A seven meter long barrel isn't that unreasonable, considering that the King Tiger already had a 6.25 meter gun. You can do better, Germany.
Yeah, now that's what I'm talking about! This was one of the proposed guns for the Typ 205 tank.
This isn't really a gun for a tank anymore, but with a barrel that's nearly 11.5 meters long, it's as big as one. Read more about it here.
The very same gun mentioned in this document. Originally a railroad gun, it was given a chance at increased mobility when the E-100 project didn't go quite so well.
Scans by Yuri Pasholok.