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.
1978, “Brass Target”
This is definitely the one. Identical scenario.