2. If, as has been alleged, that some people on the site used the guns that are also used on the movie for live fire, that would a huge violation of safety and culpable negligence. It is interesting that they were using weapons that were capable of loading live ammunition, since Hollywood guns usually have "short chambers" to prevent live ammo from being chambered.
3. Revolvers are very easy to examine if their cylinders loaded with live ammo: all anyone had to do was look at the face of the cylinder (with the barrle pointed away from your face) with the hammer at half-cock to see if there were any bulleted cartridges in the cylinder.
4. Has anyone considered the likelihood that as disgruntled, just-fired union technician intentionally loaded that pistol and placed it into the cart tp "get even"?
For camera close-ups, the gun was possibly filled with dummy rounds, that may not be distinguishable from live rounds with an external inspection.
To remove and inspect dummy rounds and distinguish them from real rounds is not a task that an actor on a set would be trusted to do, or be trained to do.
As someone else said, those could've been tested by attempting to fire each round into the ground. Nothing should've happened if they were dummy rounds. I'm also guessing that is not common practice though. If the actor wants to inspect a firearm, it sounds like the protocol is to have the armorer come to him to respond to his inquiries and demonstrate that the gun is safe.