How did you arrive at "the shallow back wound isn't even physically possible"?
To answer both you and Walt, impact velocity is inversely related to chance of perforation (so lower velocity is associated with increased chance of going straight through). This is because the higher the velocity at which a missile travels, the greater chance it has of fragmenting (it?s kinetic energy surpasses what its own mass can take). The lower the velocity, the lower this likelihood. Even at half impact velocity, the 6.5 bullet would?ve still gone clean through Kennedy?s torso.
Walt?s suggestion that some alternative missile resulted the injury doesn?t stack up as we?d need to find something weak enough to only go through less than finger?s length of relatively soft muscle tissue, but somehow strong enough to leave ordinary bullet holes (in the back and clothing) and not be taken out by the wind.