I really suggest watching "JFK: Smoking Gun," before learning too much about what the documentary offers. If it proves to be just a bunch of falsehoods and amateurish stupidity, then at least it is entertaining. The #1 reason to watch a movie: entertaining escape.
Here is the gist of the movie:
Lee Harvey Oswald DID shoot at President Kennedy using Mannlicher-Carcano rifle firing 6.5x52mm full-metal-jacket ammunition. Oswald fired two rounds: one bullet hit the pavement, and the other one struck Kennedy and Governor Connally.
The second bullet that is often been referred to as the "magic bullet," because it was proposed that it had to have taken a circuitous route, because Connally's jump-seat position in the car was not taken into account. The third shell casing that was found in the book depository was actually a "breach plug (spent shell casing)" that Oswald had used to fill the chamber, when the rifle was "unloaded."
Kennedy's head wound was caused by an accidental discharge from an AR15 held by a Secret Service agent riding in a following car.
The Secret Service violating the chain of evidence multiple times during the homicide investigation, commissioned manufactured evidence, and was protected during the Warren Commission investigation by a "softball" treatment of its involvement in Kennedy's death.
Is that theory plausible? Well, the discharge of a 5.56x45mm round from an AR15 would explain the explosive wound in Kennedy's head, because 5.56x45mm tends to tumble and fragment when it penetrates flesh, and is more likely to "dump" most of its energy within a body. The Mannlicher-Carcano round is apparently more likely to go right through a target and stay intact. Also, the smell of gunpowder was detected by witnesses on the ground. It makes sense that the Secret Service would run roughshod over a murder investigation, if one of its agents was involved in the President's death. And that theory does not negate the plausibility that Oswald was involved in a conspiracy to assassinate JFK involving Jack Ruby, Cubans, the CIA, aliens, and the Mafia. A bungling conspiracy that almost failed save the bungling of a Secret Service agent with a revolutionary, space-age, military-style rifle is still plausible.
Now, to find out more about that theory, and the people behind it - and the people who detract it.