Let's take a look at the rounds since that seems to be where a lot of the "Theories" come from....
First the "pristine" magic bullet, first off it is not pristine as many would like you to believe, it is actually deformed just as one would expect a heavy medium velocity round to look... The bullet itself is a round nose FMJ with an open core base (this means that it is not a true FMJ the base has exposed lead).. The conspiracy buffs would like to believe the crazy trajectory that we all saw in the Movie "JFK" this just isn't true... When you look at the true trajectory with the Limo jump seats being lower and to the inside like they actually were, then the trajectory straightens right out...
Contrary to popular belief this combination in the ballistics create the exact one necessary to accomplish both shots...
You have a 160 gr round nose bullets traveling at just over 2000 fps it could very well have caused all the wounds of the "Magic" bullet and the head shot..
Take a look at the head shot again and you will actually see what happens and don't forget that the car is moving away from the shooter...
As the bullet hits it will give a slight push to the head forward, and as it exits the snap back that everyone thinks is a high velocity round striking from the front is actually the jet effect from the blood and brain matter exiting the head wound.. sorry for the graphic description...
Now don't by any means think that I am saying that I believe there was not a conspiracy, but when you really start looking at all the facts you have to "Look" at them all, not just one side....
Now lets look at the grassy knoll shot, many people believe that if this occurred, the shot was actually taken with a Remington XP and a .221 Fireball was used... But let's put in any high velocity .223 bullet
These rounds will punch a small hole going in and do one of two things when all that kinetic NRG is dumped... If it is an HP round they explode with quite dynamic results, if it is a FMJ they either go through and leave a huge exit wound or punch straight through leaving a .22 caliber hole in and out... it really depends on what the bullet upset is as it travels through the target... Any varminter will tell you some shots give explosive results "The Red Mist Club" and others fired from the same gun using the same bullets and the same range barely leaves a hole....
Now there is a theory floating out there, done only on the ballistics of the Kennedy assignation that the fatal head shot came not from Oswald nor the "Grassy Knoll" shooter.... But the cover up and the head shot came from an accident in the Secret Service car behind the presidents... That an accidental discharged occurred from the early model CAR-15 that was in that car had a bolt slam fire and caused the cover up and the fatal head shot.... I have the book gotta look for the title and author interesting read...