I think that may also be due to the different shape and construction of the rounds. Mass x velocity, absolutely, but pistol rounds tend to be big 'ole dumb slugs of lead with about the same aerodynamics as a brick. Centre-fire rifle rounds tend to be much pointier, even if they aren’t proper AP or SLAP. That makes the point-loading at the point of impact much higher, and the round holds together a bit longer before mushrooming out.
On the other topic - he could have done the same thing with big balls of frozen boiled water (boiled to get rid of the bubbles). Then all those shards just melt away.