The one operating the weapon is ultimately responsible for knowing how to operate the weapon AND whether or not the weapon is loaded and hot. Baldwin was the operator. He could have chamber-checked the weapon in less than two seconds.
Ok, but I would think that's far above the ability of all clueless, ditsy Hollyweird actors and actresses. I'd tend to put the blame on the prop people whose one and only responsibility is to ensure no live rounds in the guns. The prop department could have two or three people responsible for independently checking, double-checking, and triple-checking before handing the gun to the ditsy, clueless Hollyweird actor who has to worry about memorizing their lines, when to act and speak, where to move, how to interact with the other actors, etc.