With an elevated 6th floor shot coming from the left of Kennedy, penetrating at an angle sufficient to blow out the right side of His skull how did a large portions of his head fly almost directly left , up and over his head, come to land on the far left drivers side of the trunk where Jackie grabbed it ?
Looking at Frame 313, it is clear that the debris was not propelled directly away from the shooter in a narrow jet. Instead, debris was scattered in the a wide 3-D arc. Some propelled upward, some downward, some to the left and some to the right. Some of it even went pretty much straight up. Look at frame 313.
A small portion of the brain, like an inch across would be small enough that if it went upward could be effected by the relative wind to the limousine, 18 to 23 mph, to blow back onto the trunk.
You are using "Arm Chair" mental experiments. Not "Real Word" analysis where we observe the results of a real bullet strike on an object, like a taped melon or a human head. Forgot about "Narrow Jet directly opposite the shooter's position." Think instead of "Wide 3-D arc for debris scattering."