Fresnel
I think that was
Shoot 'Em Up, but I think a .50 could do it with properly reinforced bars. Those things carry a lot of power downrange, and even my SMLE can give a 30lb plate enough shove to move it a foot or two.
The problem with my first view of this episode was I had to take a phone call, which turned into a hella long family video conference call. I didn't get to watch the full context.
What they did explain about the first round of testing was that the bearings were not designed to free wheel, making the merry go round 'safer' for play. It was fun to watch them punch holes in the safety bars with a .44 magnum.
But they learned to add a sandwich of two 1/2 inch plates and weld them onto the bars. It was even more fun to watch the first round of .50 cal testing when one of the 1/2 inch steel plates was blown off by the impact! As I recall, that shot managed to push the MGM over 8 inches with the factory bearings.
That's where I had to leave the show. You can see the post episode Q&A online at this link: http://dsc.discovery.com/videos/mythbusters-firearms-force-aftershow.html#mkcpgn=twdsc2