We all know there are four redundant rules that will keep you safe. I'm not going to repeat them. But, on a film set, they often violate at least the first two on a routine bases when filming scenes. That's why it's so darn vital that a prop master be on hand to double and triple check that the gun is unloaded at all times. That the gun never get's passed from person to person without being repeatedly checked. AND THERE SHOULD BE ABSOLUTELY ZERO LIVE AMMO ANYWHERE ON SET EVER! And, if the gun is ever to be used with blanks, that the barrel is visually inspected to confirm there are no obstructions at all that could turn into deadly projectiles when the blanks are fired.
Also, if dummy rounds with bullets are used in a revolver so it looks read from the front, they have to be real dummy rounds, not just dummy rounds made by removing the powder, leaving the live primers on and jamming the bullets loosely back into the cartridge. Like what happened with Brandon Lee when he was killed.
Further, no gun loaded or unloaded gun should ever be pointed at an actor or the camera/cameraperson unless the scene specifically calls for it. Even then there are clear bulletproof Lexan sheets that can be used for extra safety. When shooting such scenes an armorer should be there to immediately check and confirm the gun is unloaded prior to the shot, then immediately take possession of the weapon from the actor as soon as they call "cut!" And, when the actor first takes possession of the unloaded or blank loaded weapon, they should know how to confirm the condition of the gun rather than simply taking the armorer's word for it.
It sounds like they got lazy and assumed someone else had confirmed the gun empty because that was there job. At then the gun was left lying on a cart where who knows who loaded it with live ammo for who knows what reason. Again, live ammo should never be anywhere near a set.
Here is a great interview with Dean Cain explaining safety protocols on the set when firearms are involved.
Damn, regardless of your politics and/or love for Mr. Baldwin, a young wife and mother lost her life and another person was wounded. As firearms enthusiasts and compassionate human beings we owe it to those involved to try use this terrible tragedy to spark a wider conversation about gun safety. Guns don't kill people, but if you don't treat them with respect a careless individual very well may.