I was wondering if the article I found can be true.
Unfortunately it's in German.
http://www.wfg-gk.de/warum36.html
But to bring it to a point; it says the Abrams can't fire while moving because the gun-tube is to heavy (50% heavier then the Rheinmetal L44 or L55) so the stabilizer (NATO standard) is to weak.
Does anyone knows more about that?