Here is exactly how Brandon Lee died. It was a revolver and they wanted to see the bullets in the cylinder during a shot of the gun pointing towards the camera. To do so they took live ammo and removed the powder, but left the primers in place, effectively making a bunch of squib loads rather than true dummy rounds. Someone fired one and the primer went off and propelled the bullet into the barrel of the revolver where it got stuck.
The gun was then used for another scene without checking that the barrel was clear. I guess nobody wondered where one of the bullets had gone from the homemade squib rounds when they unloaded it. With the bullet still stuck in the barrel, the gun was then loaded with blanks and fired at Lee from about 15' away, which shot out the stuck bullet and killed him.
Pardon the discussion of unmentionable. My understanding these days is that many movie productions will not use any kind of blank rounds or guns capable of actually firing if the shot requires shooting at an actor. They would rather spend the money to add in the gunfire digitally than risk injuring an actor.
I'm not sure what happened with Alec Baldwin, but one story I read said that three guns were on a crafts cart, one was taken and given to Baldwin by someone who yelled "cold" as they did so to indicate to everyone on set that it was unloaded and safe. Apparently not.
There are also reports that some of the production crew walked off the set before the shooting ever happed for various reason, including possible safety issues with the handling of firearms on set. It gives me the willies just to think of being handed a gun and trusting that it was unloaded without verifying it personally. And doubly so if I'm going to be pointing it at someone or have it pointed at me.
How do you live with yourself knowing you killed a friend/coworker out of carelessness.