It's not mass, but energy, I think you mean. And they explored this on Mythbusters a while back too. The conclusion was it was unlikely a free falling bullet would be lethal.
But I can imagine a missed shot, because he wasn't likely shooting "straight up" or even nearly so, remaining in a ballistic trajectory until it hit something, and it's unfortunate the child was in that path.
I'm not going to be judgmental here, not knowing all the circumstances. It certainly appears there was a negligent use of the weapon here, though it's not hard to imagine the mindset that would have allowed it.
The lesson, be certain of what's beyond your target, probabilities are not reliable. Oh - and use the proper weapon and ammunition.
Amen.
One of those forensics shows on cable looked at a case of a kid at an air rifle competition who suddenly dropped dead. They found a 9mm bullet in his brain. He was at an indoor range surrounded by people, and no one saw a thing out of the ordinary. There were no firearms -- just air rifles -- in the building.
The investigators found the hole in the skin of the building -- basically an aluminum quonset hut -- and used lasers to track the bullet back to its origin. They concluded that someone at the nearby outdoor range was a lousy shot, or his weapon had double-fired -- the range was not up to spec, and the stray round passed through a tiny gap between the earthen berm and the wooden baffles where they should have overlapped.
Lesson #1: One-in-a-million does not mean impossible. If you're talking about a firing range where thousands of rounds fly each day, calling an event "one in a million" is just a measure of how often it will happen. As they say, if you're a one-in-a-million kind of guy, there are 1,200 folks in China just like you.
Lesson #2, as you point out: always, ALWAYS, check your backstop.
Kinetic energy is mass times speed squared. So the mass is relevant, especially as Lou specified, *at terminal velocity.* Plus, a projectile with a higher inertial mass has more oomph to overcome wind resistance, and will therefore have a higher terminal velocity.
Mythbusters tested the falling bullet myth with everything from a .22 to a 9mm to a 30-06 (M1, I think); I wouldn't like to be hit on the head with a falling .50 cal. It might not be lethal, but I bet it would leave a mark.
In simpler terms, if you're on the sidewalk and I'm gonna drop something on your head from five stories up, would you prefer a tennis ball or a bowling ball?