The Santa Fe County Sheriff's Department has spent the last year investigating how live rounds made it onto the movie set. Armorer Hannah Guiterrez-Reed and assistant director Dave Halls were the only other crew members believed to have handled the gun that fired on set.
Early reports right after the shooting said that staff and actors were using this weapon or others for live round target practice during breaks and downtime. If true, this would certainly explain how live rounds were brought on-site. Also, this would be an extreme dereliction of gun safety rules and guidelines.
There were also early reports of people using the prop guns as a joke, pointing and pretending to shoot at each other. This follows my train of thought. I think Baldwin shot at the crew intentionally as a prank/joke to scare them, thinking it was only a blank. The shot is way too accurate to be chance. Of anywhere in the room he could have been pointing, he just happen to be aiming at two of the camera crew when the gun "fired itself"? I doubt it.
Since the shot occurred during a camera 'test', there was no need for any rounds whatsoever to be in the weapon, including blanks. In fact, there was no need for the revolver at all. The whole situation is unnecessary and unexplained.
Of all the articles I have read about this, one crucial detail is missing. How many rounds were loaded? Were they all live rounds? Or was the one fired the only live round in the cylinder?