My opinion basically agrees with that report by Brian and Kenneth. There are a lot of other reactions, etc from the film makers, the crowd, etc that suggest an early shot. Why it missed will likely remain a mystery. However I suggest that it could have been fired inadvertently before it was fully aimed. There is potential interference from the box in the window, the electrical conduit pipe next to the window. Plus, the trigger was tested to release at a much lower force than the lowest safe force the military uses for that type of rifle. Premature shots can happen to even the best marksmen, especially the first time they put their finger on the trigger. I find it hard to believe LHO would miss the entire limo unless it was an inadvertent shot that happened before it was fully aimed.