Great analysis and inspection by you both, macsak and 230rn.
Unexpected things do happen in high velocity impacts and ejecta can come back opposite of impact. But the comparison to water or meteorite impacts doesn't explain the bullet's behavior in the video. The bullet (or at least a very substantial fragment) comes almost straight back, what, about 300-400 yards? I've seen many up close super slo-mo vids of various rounds, including 50 cals, hitting steel plates 90deg and obliqued, and the jacket obliterates with maybe 1000's of fragments deflecting at various angles, and with the lead core acting as a liquid like the water droplet or meteorite ejecta simulation, but none of these vids ever showed anything like what the bullet in the vid appears to do. IOW, thousands of fragments doing various things, but never a substantial fragment returning in the original direction of travel with a velocity necessary for traveling hundreds of yards in a second or two. I guess it can happen. Just hard to visualize the whole thing.