I refuse to defend baldwin or anybody on that set who had a chain of custody, I wholly believe there was negligence, however there are a few possibilities that do exists but are so unlikely that I don't believe them myself. The more likely of the two is that the propmaster/team were deeply ignorant of these weapons, failed to educate baldwin, or he failed to learn, and that somebody loaded it with real ammo for what was likely a photo shoot because they were lazy and didn't want to unload the primers and powder to make safe dummy rounds for prop purposes and through their negligence or the impropriety of the set coordinator, assistant/producer/ assistant directors or just a gofer they retrieve this loaded gun for a photoshoot, this is likely what happened, the unexpected element here might have been that live rounds got mixxed into a box off dummy cartridges by extreme negligence, loaded into this gun and the gun being set on a cowboy carry was given to baldwin and he over extended the action and cycled off the empty chamber and let the hammer slip when he was cocking or decocking it. The second extremely unlikely mod is that this is a really old prop gun from an old cowboy serial or it was bubba'd to shit and back to make it a fan fire only gun, I'm aware that you can partially disassemble these firearms and fool with the inner working a tiny bit to make a gun that just operates on the flat spring tension in the hammer and the hand of the gun moving the cylinder, take the trigger out of the equation by literally removing it and removing any other safety function. it's reported this was occasionally done by a handful of outlaws who just wanted a high volume of fire. a gun like that doesn't need to be on a set but if the previous prop master walked it's conceivable that her replacement wouldn't disassemble all the guns and discover this gun's true nature, especially if the trigger linkage were missing but the trigger were still present and somehow under tension. Like I said I don't see the second one being a possibility, it's more likely the weight of the gun and his thumb slipping on a cocking action made him instinctively grab the gun tighter and then he pulled the trigger, his inexperience with firearms not being addressed is the fault of the producer, he was also the producer, the presence of live ammo on set was almost certainly a consequence of his actions as producer, in hiring the wrong people or pushing them too hard for time.