Knowing your great love for the legal system, Nanobyte, it is unsurprising that you have, once again, managed to present a problem of some complexity and interest. 
I don’t know the law in Michigan, so if someone does, and can correct any errors which may follow, please feel free.
Unless there’s a specific statutory or caselaw definition to the contrary, involuntary manslaughter, as a matter of common law, arises from, inter alia, the failure to perform a legal duty expressly required to safeguard human life or from the commission of a lawful act involving a risk of injury or death that is done in an unlawful, reckless, or grossly negligent manner.
I suspect there is, in Michigan, a legal duty of care with respect to safeguarding firearms, so that the failure to perform that duty may give rise to a charge of manslaughter.
But here’s the interesting part – there is also a law, quoted by Nanobyte, which provides that in the specific case of a firearm being negligently discharged by a person, which kills another, the offense is a misdemeanor. So, Nano reasons, how can this make sense? You shoot someone yourself and it’s a misdemeanor, you leave your gun out and it’s a felony.
Unfortunately, the legislature has the power to do just that. They may impose a more serious punishment for what may seem a lesser crime. They may even impose a greater punishment for a lesser-included offense.
I remember a case a while back in which gay men were targeted for soliciting sex outside public park restrooms.
In the jurisdiction in question, sodomy was a felony, as was offering to commit sodomy. But offering to commit sodomy for money, prostitution, was a misdemeanor.
One man convicted of a felony for offering oral sex to an undercover officer appealed his conviction, pointing out that if he had offered the oral sex for money, he’d have a misdemeanor on his record; because he stopped short of doing that, and offered it for free, he was now a felon.
No dice, said the appeals court. The legislature has every right to define and punish crimes, even in the inconsistent manner here.
So, too, I would imagine, in Michigan.