Another source
https://www.cbc.ca/news/entertainment/prop-guns-movie-sets-1.6221637
What are the rules for firearms on set?
The weapons master is required to be on set whenever a weapon is being used. The Actors' Equity Association's guidelines state that, "Before each use, make sure the gun has been test-fired offstage, and then ask to test fire it yourself. Watch the prop master check the cylinders and barrel to be sure no foreign object or dummy bullet has become lodged inside." Further, "All loading of firearms must be done by the property master, armourer or experienced persons working under their direct supervision."
"Nowadays, all weapons are checked before your blanks are put into the weapon.… The blanks themselves are never loaded until the very last minute, when all crew is in position, so the armourer knows exactly where every member of the crew is so that no one's walking through any danger areas the armourer has set up," said armoury co-ordinator Sam Dormer.