Ah, but watch “Brass Target” !
In the movie, it goes down this way - since the location of the accident is pre-determined, a low-velocity silenced gun is built to be used to fire a pre-formed projectile made to look like a truck rivet. The asassin fires at short range in the post-accident confusion.
Of course, this just means that the story-telling “Ex-OSS” guy took his tale from a film (and/or the thriller novel it was based on).
..... another possibility, admittedly far-fetched, is that the screenwriter of “Brass Target” was familiar with the events surrounding Patton’s alleged assassination.
You never know!