I'm a little confused as to the rules for a Breakthrough gun in Flames of War. Unlike the bunker buster it doesn't have a dedicated section in the rule book that I can find. It is mentioned as not allowing infantry saves like a bunker buster, but the rule book doesn't state if it follows the other rules for a bunker buster.
The Breakthrough Gun rules are not covered in the Flames of War 2nd Editon or earlier rule books. This is supported by the comments in the FoW 3rd Edition Q&A:
There is a small blurb on the Battlefront Minature's Flames of War website.
An example of a unit with the ability is listed just above this on the same page:
The Hammer and Sickle supplement also mentions the Breakthrough Gun rule. It also fails to mention any additional information about the rule, which leads me to believe that there isn't much more to the rule (although it isn't definitive proof without access to a 3rd Edition rule book, or the Know Your Enemy publication.