I think you mean armorer, which is a different role to props master, but that's who most people seem to want to blame.

But it needs to be emphasised that the armorer, Hannah Gutierrez-Reed, was hired as both armorer and props assistant, and days before the shooting a line producer had told her off for dedicating too much time to weapons safety and not enough to assisting the props master. She pushed back, complaining about the lax gun safety on set, but was overruled. So on the day of the shooting she was elsewhere, assisting the props master, as she'd been told to.

It was the AD who decided to go ahead with an unscheduled rehearsal that involved a gun without calling for the armorer. He took the weapon, declared it cold himself, and handed it to Baldwin - none of which he should have done, and he should have known that. So he's definitely negligent in that sense but obviously a bunch of things went wrong leading up to this - not least bad management. I mean who hires a part time armorer on a western?