As I understand bullet ballistics, entry wounds are smaller than exit wounds. Given that, what I can’t square in the Donahue theory is the final, mortal wound: going by the Zapruder film, it appears that the bullet entered the front of the head because bone and tissue fly backward, away from the front. If that wound is supposed to have hit the back of the head first, I would expect that material to be flying forward.
The theory also begs another question: was every round from that Secret Service agent’s weapon accounted for when it was returned to the armory?
Googling “Frame 313’ brings up the critical instant from the Zapruder film. There appears to be a ruddy explosion at/on Kennedy’s forehead. If that’s the exit of the bullet taking place, then it answers my question. The entry was from behind. That opens the door for Donahue’s theory, but does not answer whether that agent’s ammunition was finally accounted for. It certainly supports the firer being Oswald as he’s the only one we know for certain who fired any shots.
Here's what the article says, toward the end:
As far as we know, Hickey was the only agent carrying a long gun that day, and at the hospital he was later instructed to return the weapon to the trunk of the follow-up car, according to his Warren Commission statement.
The article also quotes hospital staff as saying two Secret Service agents burst into the hospital, and one was holding a "machine gun." When an FBI agent ran in, the Secret Service agent with the gun knocked him out cold. Then the article raises the question: Were the agents trying to keep the FBI from taking possession of the gun? (Interesting theory, if the story is true.)
I thought about that too. If there was a cover up and the agent did accidentally fire his rifle, then the returned ammunition and rifle would have been part of the cover up.