http://www.skepticwiki.org/index.php/JFK_Assassination
In the Zapruder film, starting with the third shot at frame 313, the upper right part of Kennedy's head blows out forward, and his head lurches back and to the left. According to the conspiracy theorists, there is no way this shot could have come from the book depository; it had to have come from the front, perhaps from the grassy knoll. The reason why, they say, is that the bullet entered from the front of JFK's head and pushed his head backwards. A shot from behind, they tell us, would have propelled his head forwards.
This is completely wrong in every possible way. Bullets do not make such a splattering when entering, only with the exit wound. Frangible bullets would however produce this effect.
Also, there's no reason why a shot from the rear would move the head forward. It's a classic image in movies, where the hero shoots the bad guy and the bad guy is thrown backwards, but it just doesn't work. In Episode #25 of Mythbusters not even the most powerful firearms pushed the human-sized target in the direction of the shot. This would not be expected as all the shots were aimed at the dummy torso and not the head. The reason is obvious if you understand basic Newtonian physics: since for every action there is an equal and opposite reaction, a shot that pushes the target in the direction of the shot must push the shooter back by a comparable amount.
Actually, that same Newtonian principle explains why JFK's head would have moved towards the shooter. The bullet doesn't meet much resistance as it enters and is going pretty much the same speed when it exits, but as it exits it also takes lots of pieces of skull and brain with it. This makes a kind of jet blast which propels the head in the opposite direction. This effect is easily confirmed by wrapping a melon in one-inch fibreglass tape and shooting it with a high-powered rifle. The melon actually moves toward the shooter. This demonstration was devised by Dr. Luis Alvarez, a friend of JFK, and was demonstrated by Penn and Teller in their book How To Play With Your Food and again on an episode of Bullshit!
The "Pristine Bullet"
Conspiracy theorists say that the second bullet, the one that struck both JFK and Connally, was far too pristine to have struck both men, and could not have been shot from Oswald's rifle. In Oliver Stone's film JFK, he accuses Jack Ruby of slipping the bullet into the hospital stretcher.
The problem with this claim is that the bullet isn't pristine at all. It has the flattening one would expect, and the markings and scratches on the bullet prove forensically that it was shot from Oswald's rifle.
Lee Harvey Oswald
His records show that on two occasions he scored 48 or 49 out of 50 when firing rapidly at a target twice the distance of Kennedy from the book depository. He qualified as a sharpshooter in 1956 by scoring 212 (2 above the minimum for qualification). In 1959, he only qualified as a marksman scoring 191 (exactly the minimum for qualification). In these three years in the marines, his rifle skills had deteriorated.