That's one of the reasons I want to find the video, so I can nail down exactly what they were using and at what distance.
It was longer barreled, and vaguely 50' away, but probably not a 1700s musket. They were definitely using a lead ball, not steel or iron. The ball struck lower right IIRC, and glanced off, leaving a shallow round indentation.
It was within the last year and it had zero sensationalism so it couldn't have been most of the "science" or "documentary" channels at this point (it's amazing how far down hill the iterations of Discovery Science have gone in the hunt for cheap programming and higher ratings).
E: Finally found it, not Smithsonian... PBS Nova. All instances of the video I can find are behind regional blocks or paywalls. But the preview/intro does briefly show the firearm used and the distance.
https://www.youtube.com/watch?v=S9cWlsbX894 They also discussed the big difference between cheap untempered plate and high-quality tempered steel plate in resisting the heaviest hits.