Sergeant Nicholas Ranstad broke down the shooting in minute detail based on his raft of experience around high-powered weapons and long-distance shots.
www.dailymail.co.uk
Behind a paywall, but it's an interview with a former army sniper. He says that he thinks this guy was a bit of an amateur, as he was likely aiming for Charlie's head and hit his neck instead (nerves may have gotten the better of him).
Thinks it's someone with range experience, and they should look into people who have been at gun ranges.
Doesn't think it's an AR type rifle, but a bolt action (based on the sound).