Not only was his research lengthy, it was very thorough. He even showed errors in other experts’ analysis.
Boiled down - the type of bullet Oswald shot was not capable of causing the massive head wound that blew his head apart. The bullet that went through the neck area was consistent with Oswald’s bullet. Donahue explains why a different type of bullet shot from behind did this, contrary to the popular accepted explanation that a shot from the front blew out the back of the skull.
Find a copy of the book, you’ll be glad you did.
But you did not answer Fester Chugabrew’s question; “Is there positive evidence the projectiles differed in caliber? By that I mean is there a available for inspection today a projectile known to have pierced JFK that could not possibly have been fired through Oswald’s rifle? That would go a long way in confirming Donahue’s scenario”.
Took a look at the book and some of the reviews on Amazon. There are some ways in which Donahue’s efforts could clear the air.
One would think the aural experience of those proximal to the event would easily hone in on differing sound and location for each shot fired. A shot fired from a building 100 yards away elevated would sound considerably different than one fired from a car passing 25 yards away on the ground. Maybe Donahue’s work explores and explains that angle. I would think the result of a bullet passing through soft flesh vs. a bullet striking the skull would differ considerably as well in the way of fragmentation. Just my amateur thoughts.