When shooting a head it may get sucked towards the bullet. This has been demonstrated with water melons and cellophane tape(soft/goey insides, hard shell, somewhat like a skull). The bullet slips through imparting little momentum, on it's way out it sucks out a bunch of goo that imparts momentum to the object in the opposite direction. The momentum imparted is small in either case, but it's not nescessarily in the direction you think it ought to be and without further considerations the movement of JFK's head can't be used as hard evidence for anything.
The magic bullet went much straighter than most pressume and the discovery channels JFK: beyond the magic bullet recreated the path of the magic bullet using a bullet of the same batch, a vintage carcanov rifle, ballistics dummies and a trained marksman pretty successfully from a distance and direction similar to that of Oswalds. This pretty shows that it's plausible for a single bullet to have done this much damage. Dale K. Myers reconstructed the scene in 3d from the zapruder film, city layout and other sources and showed that the wounds were consistent with 2 hits from a narrow range of windows in the book depository where Oswald was located. Neutron activation analysis rewieved and conducted on the bullets and bullet fragments found in JFK and connelly by Dr. Vincent P. Guinn is consistent with two bullets fired by Oswalds rifle.
Tests conducted on the same make of bullets from the same rifle as used by Oswald have shown that it is possible to obtain intact and relatively little deformed bullets when slowed down by animal tissue or balistics gel before hiting bone, explaining the shattering of the bullet that hit JFK's head and the relative intactness of the 'magic' bullet.
I'm convinced that a second gunman is unnescessary to explain what little evidence there is.
