This is the premise of Donahue's theory. It makes sense based on the evidence: (1) 3 spent 6.5mm Mann Carcano cases, with one primer exhibiting multiple strikes (a dry-fire case perhaps). (2) the diameter of the skull entry is too small for 6.5mm, but matches that of a 5.56mm projectile. (Skull tissue doesn't "snap back" -- it's bone).
The grassy knoll theory's major weakness is that this not a shooting position a marksman would choose. The target is would be moving laterally left->right which is hard track. Before or behind present a relatively stationary target to the shooter. A shooter positioned in front would have to deal with the windshield. It was intact, so that position is ruled out. That leaves the rear -- the book repository.
I was convinced when I first read this theory 28 years ago. Oswald fired two shots: the first missed, but the second struck Kennedy in the back and the 160gr FMJ bullet easily passed through him into Connelly. The leaves the dramatic third shot which struck Kennedy in the head to explain. A 5.56 NATO round fills the bill. Who has them? The secret service agent standing in the rear seat of the follow-on limo was sporting one. Donahue's experience investigating shootings positioned him perfectly to address what is or is not possible.
Oh, Donahue beat Oswald's supposed feat demonstrating that it was not beyond the skills of most marksmen. If we assume that Oswald fired three shots, then Donahue and another shooter in that CBS reenactment bettered Oswalds time. (But then Oswald only fired twice according to Donahue.)
Well, Donahue mentions the errant shot that missed and hit the road, then the shot that hit Kennedy in the back.
His evidence of the third shot being from the SS agent and the subsequent bullet fragments in the skull have not been disputed (the fragments in the skull).
I'm not privy to any information relevant to a third shot supposedly fired by Oswald..or what was the evidence that supported it.