It was a prototype tank that had a 140mm gun. To my knowledge, the US army stopped pursuing the project and the Thumper project never went beyond the prototype phase. I believe they felt they no longer needed a 140mm gun on their MBTs once the Soviet Union fell and could no longer fund developments such as Object 477, 195, or 490.

Still quite a crazy tank! It would’ve been fascinating to see how it would’ve been if it had been the main tank for the US army. I wouldn’t want to be on the receiving end of a 140mm HE round!!