M16
I know that the M16 rifle was first made in 1963, but I have doubts about the USSS and the FBI being equipped with such rifles at the time of JFK's assassination....
shareThe USSS was equipped with AR15 (M16s). See here
http://www.csmonitor.com/The-Culture/TV/2013/0729/John-F.-Kennedy-assa ssination-Did-the-Secret-Service-agent-do-it
George Hickey mentions his AR15.
I have my doubts.
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If I've never seen it before, it's a new release to me!
Not sure if you mean you have your doubts about whether the Secret Service and FBI had M16s, or if you have doubts that a Secret Service agent accidentally fired the fatal shot.
If it's the former, there are photos of the agent in question in the Cadillac behind the presidential limo, holding the rifle, after the shooting as they are speeding toward Parkland. There's no mistaking that distinct signature triangular front sight mount.
As for the latter, there is no photo or movie taken of the Cadillac at the moment of the fatal shot that shows the M16 anywhere, let alone in an unobstructed line of sight to Kennedy's head.
In 1993, for the 30th anniversary of the assassination, Newsweek did a sidebar article on the kookiest of the conspiracy theories, the last two being named "Honey, I Shot the President 1 & 2". Number 1 was the aforementioned theory about the Secret Service agent with the M16. Number 2 was that Oswald did the shooting but had no intention of hurting President Kennedy at all; he was trying to kill Governor Connally because when Connally was Secretary of the Navy, he was the one who changed Oswald's discharge from the Marine Corps from General to Dishonorable after his defection to the Soviet Union. JFK just happened to be in the line of fire!