I don't understand how a Special Weapons and Tactics officer can miss a shot so badly. Only thing I can figure is he's mistakenly had his weapon on 3 round burst, as it's been reported 3 rounds were fired, but even then a trained shooter would have the first round on the mark, especially under 50 yards.
Ridiculous.
Something funny is going on and lies are being told. Even if he didn't intend to have the rifle on 3 shot burst, a SWAT member will still put all 3 rounds into the target, at a close grouping. I've seen video of a SWAT team member, on a gun clubs range, emptying a mag on 3 shot burst and the weapon hardly moves.
Personally I'd say its an accidental discharge, which is why the lad sitting on the floor wasn't hit and Kinsey was only hit the once. Like you say, there is no way he'd have hit Kinsey in the leg, even with a handgun, never mind a rifle if he was aiming at the autistic lad. Would also explain the "I don't know" when asked by Kinsey why he shot him.
