A ballistics expert (now deceased) came to that conclusion about the fatal head shot coming from a secret service agent’s AR-15. The book “Fatal Error” by Menninger goes into detail; quite convincing.
My theory is that the sealed records won’t confirm the fatal shot from the SS because it would contradict the Warren Commission’s conclusions - based on selective use of evidence to support the pre-ordained givernment narrative that Oswald acted alone and fired the only gun. To keep such records to show the contrary would damage this country’s feral givernment even 60 years later that is incomprehensible. The givernment is the top expert bar none at creating forgeries, destroying documents, denying anything ever existed ... etc. The truth won’t be found beneath the redactions, nor in its files. Plausible deniability and outright lying are the modus operandi.
The book is titled “Mortal Error.” The Secret Service agent named in the book was George Hickey. He sued the publisher for libel twice, once unsuccessfully, and then reached an undisclosed settlement with the publisher upon publication of the paperback version. I was originally persuaded by the argument in the book, but I now have doubts about it.