Ok, fine. Several things wrong here.
First, if he was at all trained and conscientious he would know the four rules of firearm safety and no-one would have gotten hurt. The only way this happens is if he violated basically all four. So Alec (and greater Hollywood) you still want to keep bashing the NRA and us firearms enthusiasts? I handle a firearm every single day, and I've never had a negligent discharge, never hurt anyone with my firearms.
In addition, the production company, from the director on down to the prop guy, screwed up. I thought it was a rule on many sets that there simply is no live ammo anywhere on set. Period. If there are scenes that need to be filmed with real firearms and live ammo these are done on separate days with minimal staff/crew around and everyone knows it is a live-fire day and every weapon is treated as real. Other days are nominal/normal filming where everything is props and/or blanks. They never mix the two to prevent just this sort of thing. Guess that's not an across the board SOP for all production companies. But sure, tell me again Hollywood how I'm dangerous... :-/
Someone reported earlier that the crew went out for target practice. If that is true someone forgot to unload the weapon before it got back on set. Also reported earlier that no live ammo was permitted on set.