Yep, he decided to go ahead with an unscheduled rehearsal of a scene that involved a firearm without calling in the armorer, who was working elsewhere on set in her second role as props assistant at the time. He wasn't in any position to handle the gun, declare it "cold" or check it for safety - and he should have know that considering he's got credits going back almost 30 years.

A lot of other things went wrong - like the decision to only hire a part time armorer for a western movie, the line producer downplaying the priority of gun safety, and however live ammunition got in the gun (crew playing around with it? Ammo mixed up in the boxes?) - but the AD is definitely negligent for declaring the gun cold without calling in the armorer.