IMHO, a manslaughter charge in this incident is off the edge.
I know nothing of military practices, but I will assume the man in question was supposed to have his weapon fully loaded, with a round chambered, safety on.
The safety on this type of weapon is a lever that needs to be pused through a 90 degree arc to move from safe to fire, at least in the civilian versions. That could happen inadvertantly quite easily in an armoured vehicle bouncing over rough terrain, with the rifle brushing up against the soldiers or anything else. I have seen S&W revolvers come out of holsters fully cocked after a ride down a
highway here in NB. The guard never touched the piece.
Once the safety is off, it requires a light tug on the trigger to fire the weapon. Unlike what you see on TV, professionals should be VERY well versed in Rule Three:
KEEP YOUR FINGER OFF THE TRIGGER UNTIL THE SIGHTS ARE ON YOUR TARGET! I know my people get drilled on it ad nauseum.
But triggers do get tugged by other things.......gear etc.
I'd like to know what this guy said to the MPs in his interogations.
Even a violation of the sancrosanct Rule Three hardly rates a manslaughter charge.....IMHO.