It all comes down to our good friend gravity. When a bullet goes up, it must come down and you have little control where it’s going to land. Though a falling bullet is estimated to have only 10 per cent of its original muzzle velocity, it still has a good chance of injuring and possibly even killing you.
A 30-calibre bullet falls at 322km/h, while a 50-calibre falls at almost 531km/h. And that’s why they kill people. A bullet falling at as little as 170km/h can pierce the skin, while one falling at 217km/h can travel through your skull and into your brain.
Even if the bullet falls at a rate somewhere between 160km/h and 217km/h, it still can knock you on the head and give you a traumatic brain injury. Of course the bullet can miss your head, but it could hit you in the arms, shoulders, and legs. Ouch. [Amazing Facts and The Straight Dope]