Is muzzle velocity is 2165 ft/sec (approx.) meaning travelling the distance to JFK?s back and piercing his torso chopped off less than 400 ft/sec and you propose that between entering the back and hitting the rub it somehow lost at least 800 ft/sec. Umm? no. Just no.
no. Just no.
I'm going to need a little bit more than this before I abandon what I proposed as a possibility.
A bullet in mid-tumble could slow at a more elevated rate (because it is tumbling, of course) than the bullet would slow (not tumbling at all) when it struck the President in the back.
You're in error to compare the two scenarios as you did above. Apples and oranges.