Apparently there is this cancelled M8 "Buford" Armoured Gun System,http://www.fas.org/man/dod-101/sys/land/m8-ags.htm , http://www.military.com/soldiertech/0,14632,Soldiertech_Buford,,00.htmlalthough it should be noted that it is not considered a "tank", but it was another armoured vehicle...