I presume you're talking about the claim that has gotten recent attention, that Secret Service (not CIA) Agent George Hickey, Jr., accidentally fired his AR-15 when the driver of the chase car he was in suddenly accelerated, his shot hit the back of Kennedy's head, and that the Secret Service was so embarrassed that they conspired to hide the evidence. If so, this claim does not work for several reasons.
First, if he had fired at the exact moment that Kennedy's head exploded, his fellow agents would have tackled him as a potential conspirator and the most immediately visible danger.
Second, the AR-15 and the Carcano rifle that Oswald fired do not sound the same. Witnesses would have heard two different kinds of shots.
Third, even if they did sound the same, all the shots captured by the open mic on one of the motorcycle cops' helmets sounded approximately the same. If a shot came from relatively close and in the open air, it would have a different sound profile and volume than the others, coming from open air instead of from a restricted window.
Fourth, that would have been a one-in-a-million shot. The president's head would have been a small part of the area swept by the muzzle of the rifle.