[Does so]

four rounds fully auto at 400 yards and all four hit the head?

One round—fragmented?

[archy]

four rounds fully auto at 400 yards and all four hit the head? One round—fragmented?

More likely: a LOT of people hit and less seriously injured by pinwheeling and fragmented bullet jackets, especially if military M856 rounds with a steel penetrator insert were used. This would also account for the relatively low number of fatalities and high number of WIA.