Really? I thought heat-of-passion killing was indeed murder. Whereas voluntary manslaughter is more like when you punch someone in the face and they crack their head on a curb and die. You intended to hurt them, but you had absolutely no intent to kill.
But IANAL, and to the extent I pay attention to the law, that kind of basic criminal law isn't it.
I linked the UCMJ articles in a sibling comment. I think gp's description is correct for the UCMJ. Part of the problem IMO is that there are ~52 definitions of manslaughter/murder in the US (one for every state, civilian federal law and the UCMJ) so that answer is always highly context specific.
But IANAL, and to the extent I pay attention to the law, that kind of basic criminal law isn't it.