I don't disagree that he may be civilly liable for the safety conditions in general on the set. I just don't think that his role in this particular case amounts to criminal negligence. From what I have heard, he had every reason to think that his weapon was safe to handle and use. In order to be guilty of manslaughter, you have to act with gross negligence, meaning that you know the risk of harm to another due to your action is real and significant and yet you choose to do the action anyway. In this particular case, he would have reasonably believed that the risk in his actions was essentially none at all.

The negligence was primarily on the armourer and secondarily on the guy who was meant to confirm the armourer (the assistant director? I can't recall), both of whom failed in their basic due diligence and assured the crew and cast that the firearm was safe when it was not.