Was JFK's assassination really just an unintentional shooting by a Secret Service agent?

As we look back on the 50th anniversary of President Kennedy's assassination, one theory is being revisited, the so called "accident theory." The theory holds that after hearing the shots fired by Oswald, Secret Service Agent George Hickey, riding in a car immediately behind the presidential limousine, grabbed a Colt AR-15 rifle, lost his balance when his car stopped suddenly, and unintentionally discharged the weapon, shooting the President in the head.

"It's not sexy. It's not rife with intrigue," said Bonar Menninger, a Kansas City journalist and a leading proponent of the theory. "But for that reason, in my mind, it's extremely compelling - because it's the only theory that hues tightly to the available evidence."

Menninger is author of the book Mortal Error: The Shot That Killed JFK. He also appears in the documentary JFK: The Smoking Gun which is currently showing on Reelz channel.

Menninger became interested in this theory after reading an article by Howard Donahue, a ballistics expert who had been investigating the assassination for years. In a recent interview Menninger states:

The evidence supporting Donahue’s theory, in my opinion, is compelling: The trajectory of the bullet was not right to left and sharply down, as it would have had to be from the book depository, but left to right and shallow. The entrance wound on Kennedy’s skull was 6 mm in diameter. The bullets Oswald fired were 6.5 mm in diameter. [It’s] hard to see how you could put a 6.5 mm bullet through a 6 mm hole. In fact, bullets always make holes slightly larger than their diameter in the skull. The AR-15 .223 round was 5.56 mm in diameter. The bullet that struck Kennedy behaved not like a full-metal jacket round from a Carcano rifle, but much more like a frangible, explosive .223 round. A dozen people saw the agent with the rifle at or just after the time of the last shot. Nine people in the motorcade behind the follow-up car immediately smelled gunsmoke after the last shot.

If Donahue’s theory isn’t correct, then these facts need to be explained, and no one has done that as far as I’m concerned.

Interesting theory. Could it be true?