Directors are typically responsible for what goes on on set at an "operational" level.
Yeah, and with Assistant Directors, the buck stops with them in regards to firearm safety. They're supposed to be the last check to prevent accidents from these from happening. The AD is likely never going to be working on a movie again. It sounds like they cheaped out by hiring an inexperienced armorer who should never have been in charge in the first place, and then when previous incidents occurred, the AD failed to stop the production, which reeks of carelessness by the AD.