At 2:20, Schneider tries to cock the hammer and loses it. The half-cocked sear catches it. I’m wondering if the hammer on the actual weapon is worn. Hollywood dummies could have been abusing it. And I wouldn’t trust this film’s “armorer” to catch it.
After reading so much about single action revolvers, that's how I see this incident happening. However, why was there a live round in the barrel or even on the set? And why didn't the young armorer or assistant director double check that gun before handed to Baldwin? It was a perfect storm where so many people screwed up on set.
I've learned from others who have been on movie sets that actors are not the final check if told "cold gun". This young girl armorer not only lost chain of custody for the weapon, plus didn't supervise the guns on the cart in open view and didn't bother with a final check before she handed Baldwin the gun.
Awaiting further investigation, it seems the armorer lost chain of custody. It doesn't matter if some crew went playing/plinking with the weapon and left a live round in upon returning it to the weapons cart. Her main duty was to check EVERY arm for live rounds, dummy round, or blanks before she ever hands it over. She failed.