So what matters for a manslaughter case is proving with evidence that the defendant was aware of the risk and ignored it. That is simple enough with an armourer who mismanaged rounds and didn't do her due diligence. The very nature of her role, why she was hired, was to be aware of and minimize/eliminate the risks.

In Baldwin's case, you would have to prove that, at the time, he understood that the armourer's work (and the guy checking her work) was untrustworthy, and yet he pulled the trigger anyway. You can argue that he should have known that until you're blue in the face. But a prosecutor would have to prove that he did know that, beyond reasonable doubt. The mental state of the individual determines whether the death they caused was murder, manslaughter, or just an accident beyond their control.