If the target melon is placed on a flat surface, one can impart backspin to the target melon by hitting it below the midpoint - kind of like a cue ball in pool.
But, if the target is suspended from below by a spring or from above by a rope like a head is suspended by the neck -> no backspin.
In France, there are plenty of documentaries on WW2 shown on TV. I've seen one in the past where the SS lines up victims in front of a trench and then shoots them in the back of the head -> they all fall forward. A week or so ago there was another one on with a segment on the rape of Nanking. The Japanese soldiers had 2 victims kneeling in front of them, approached them from the back with rifles and shot them in the back of their heads -> they fell forward.
I believe in the Conservation of Momentum, not only in the film, but also in the pattern of bullet fragments in the skull X-rays.