Usually some high level of negligence or a showing that you were committing a different crime and then in that process killed a person (where the crime you were committing is a crime because it could result in the killing).

So for instance, running a red light and then plowing into a pedestrian is not murder but is likely involuntary manslaughter (you didn't intent to kill the person, but you did run a red light and the reason the law that says you stop at a red light exists is, in part, because violating that law puts others at immediate risk of being killed). Or if you drive while drunk and accidently run someone over, its involuntary manslaughter because the thing you did (drive while drunk) often results in the consequence (someone gets killed)

Here the distinction may be that possession of a firearm by a felon is a crime but is not so immediate to the consequence of killing someone. In other words, you have a firearm when you are not supposed to, but you still have to do some other willful or intentional or negligent act that causes death.

Firing the weapon into the ground could be a crime itself, but I don't know if he was charged with that. If that's a crime and he committed it (and was charged for it), then I could see manslaughter being the consequence. But here in this case i

Not offering comment on the verdict as I have not followed the case but I can see where this might have been an issue. Don't forget the facts have to be proven -- sometimes prosecutors are forced to (or will voluntarily) avoid certain charges that they cannot prove.