It wasn’t the armorer’s fault she obviously lost the gun around the set there somewhere. If the crew was shooting the pistol around the area there might be ammo tickets for .45 Colt ammunition in some nearby store. That may have been the only functional real weapon in the property room, not a stage prop or dummy.
The Armorer has one job. Keep track of the weapons at all times. She was not doing her job.
Yes, she has fault in this incident. She is not the only person at fault.
"Troubles never singly come, but march in three abreast" (Proverbial)