There were supposed to be dummy rounds in the gun.
Dummies are used for a new where actors load guns, eject live rounds (or sometimes "empties", oops), or point a loaded revolver at the camera. I can make a nice compilation of those kinds of shots if you like. Take 2 seconds and I think you can remember some. Or watch/read any interview by an actual propmaster that's come out in the last few days, most if not all have talked about how they use dummy rounds on set.
Maybe, just maybe those armorers have a process that works when it's followed, and only failed because it wasn't followed.
Another IATSE armorer and FFL holder did a writeup here:
You probably watched something he's worked on. Chicago Fire and Chicago Med are ongoing projects for him. Don't need facebook to read it, he walks you through the how and why and what went wrong.