What makes you think it was supposed to be loaded with dummy rounds, when it was loaded with two blanks and one live? Sounds to me as if someone thought it was supposed to contain blanks. It's even more bizarre if the armourer and AD couldn't distinguish between a dummy and a blank or a live round. I can see someone check from behind quickly and not realizing there were two kinds of rounds - but not noticing dummies would be beyond just regular negligence, IMO. So I think whoever loaded the blanks from the set's store of blanks thought the gun was supposed to have blanks and did not check the entire cylinder.
Since it was a real, operational revolver, leaving it off the cart altogether was an option (for a rehearsal).
Since it was on the cart, it was supposed to be shown by the armourer to the AD or Propmaster that it was unloaded (this wasn't done). Then, when handed to the actor, whoever handed it was supposed to check it in the presence of the actor (per both Propmaster and Actors Unions protocols). That wasn't done.
Then, if it had contained dummies (which it obviously didn't), everyone would know about it. Then it would have been a cold gun. Something went seriously wrong here. It's very odd, even if there was plinking on set that day, that the armourer didn't check the gun when it was loaded with 2 blanks (and the live round was behind the hammer...she didn't even check to see what was behind the hammer). It's not uncommon for that chamber to be empty and for the actor to be told that on the first shot, nothing will happen - especially during a rehearsal.
Whenever guns are used on set, there's supposed to be a brief safety meeting among all parties (including the director) to explain exactly what is in the gun, how things are expected to go and whether that day will be just rehearsal or there will be a switch to blanks (in which case the entire protocol of examining the gun is supposed to take place again, in the presence of the AD and the actor).
Guns should be locked up on set when not in use, as well. Then, there's no one putting live rounds into it except the person with a combo to the safe (the armourer).
If this has been changed sometime this morning I apologize, but the previously released affidavit said-- HGR stated, before lunch, she checked the dummies in the weapon, to make sure there were no hot rounds. After lunch the guns were brought to her (from the safe) by the prop master. Halls said HGR showed him the gun which he later admitted he only saw three rounds.
After the shooting Hall said he had HGR open the weapon and it had four dummy casings (with the drilled holes) and one casing without the hole and had no cap. That casing was obviously the live round.