5.56mm
IIRC, the novel was The Algonquin Project which was made into the film Brass Target. I think this is the one I saw, but regardless I know I saw a film about the assassination of Patton sometime around 1980 and the assassin used a large rubber projectile to smash his spinal chord. I specifically remember the demo using a dummy, ala The Day Of The Jackel's mercury loaded hollow point 22 and a water melon.
It IS a plot from a spy novel.
Can’t remember which one, but it begins with Patton’s accident, which the assassin then follows up with a low-velocity, large diameter rubber bullet to the head — precisely what this report claims