The sledge hammer was fine, it was the giant steel ball that was the issue. And it was rehearsed many, many times - the issue was that the window was slightly r

The sledge hammer was fine, it was the giant steel ball that was the issue. And it was rehearsed many, many times - the issue was that the window was slightly rolled down during the demo, making the window lose a lot of its rigidity. And was sort of a moot point anyway since the production version didn't wind up having the same "armored glass" that the prototype did.