Involuntary manslaughter means homey that caught the charges probably F-ed up, but on accident. Going back to my days of being 19 and carrying a loaded M9 all day at work (armory)... I can assure you that the maturity wasn't there yet.
$5 says if this new story is true, that safety rule #2 was probably willingly violated, and dude got really scared and knew of the other instances.
$5 more says that when I was 19, there were a couple sentries playing fast draw games with an M9 on base... and one kid got ventilated and died. We had a safety stand-around for that even though it was not right where I was at. That sounds like a possibility for these new circumstances.
$10 says that one of my fellow platoon sergeants (different company) handed his M9 to his crew chief (Corporal) in Iraqistan to watch while he went inside a building for a few minutes (inside the wire). The kids (Marines) on the vehicle screwed around with that M9 until one of them took a round through the calf. I can't remember who shot who, but I still remember their names. That was the Wild West days of Fallujah.
Etc... etc...
I've got stories for days...but we already have one Lucky Duck.
*****
I have no dog in the P320 fight, other pistols fit my hand much better.
I'm also hoping that some kid didn't get ended by his buddy, screwing around... but I can suppose a thing or two because I've seen/heard a thing or two. Hopefully the second story has an adjudicated and just ending, regardless of what actually happened.