The bullets were dropping, think about people killed with bullets shot into the air when they come back down. Bullets don't just float down.

According to this site http://gundata.org/blog/post/223-ballistics-chart/

At 1000 yards it will still have 783 fps.

783 fps? That's almost as fast as a 45 acp!

I am assuming of course that he used 5.56 ammo. 45 ACP uses a 230gr bullet and 5.56 uses either a 55gr or 62gr bullet. So as far as if I had to choose which to get hit with I would take the 5.56 of course. Remember he fired at least 1000 rounds. Possibly double that. He killed 58 and I think the injured was over 500. Not all 500 injuries were bullet wounds. Injuries from the crush to get away would have been included but with 20,000 people in a crowd and just raining bullets down it is easy to see how close to 600 people were injured or killed.