I am not a Kennedy assassination buff, but there are a few things worth considering.
1. Eyewitness memories of a traumatic event are usually distorted, and degrade with time. Ten completely honest eyewitnesses recalling the same murder will give ten different accounts if you interview them immediately afterward, and will likely give accounts that deviate from their first accounts if you interview them again later.
2. Sounds echoes, particularly in the presence of hard, reflective surfaces. I've seen the Kennedy assassination site - it's in a urban area full or concrete, glass, and steel.
3. Bullets tumble and bounce. I used to be a volunteer EMT, and my training emphasized that if you see an entrance wound, you have to strip the victim to make sure there's no exit wound, because it could be anywhere.
4. A turn-of-the-last-century military rifle such as Oswald used was designed to be used for at distances of up to 800-1000 meters. After a short-distance shot, a bullet fired from would still have a great deal of kinetic energy, even after penetrating flesh and bone.
Just a few points to ponder.
That's true when it comes to witnesses to a certain degree. But when you have so many saying the same thing then it becomes more difficult to explain away. For example no less than 44 witnesses who examined Kennedy's head swore he had a defect in the right rear rather than the right-front. If a bullet enters clean and leaves with a large gaping hole that would suggest a frontal hit as opposed to the back. In this case however I believe it was an exploding bullet that hit his right temple and caused matter to blow out in both directions.