to leave the barrel. This typically results in a gun explosion if it's followed by another live round, but since the next round was a blank: it launched the bullet that was stuck and thus struck Mr. Lee.

Following this event, different safety protocols went into effect: Separating live guns from "prop/blank guns" being the obvious. Guns are so cheap in the overall budget for a film, the fact you would even use the same gun and alternate live and blank rounds is stupid.

The whole scenario screams massive basic and fundamental safety violations. Such a tragedy.