You dismiss Aaron Zapruder, who had a view of the grassy knoll. And he was former Army. He would know what a rifle shot, going off at the level of his feet, would sound like. And they never show it but the end of the film shows nothing behind the fence. Nothing.
There are dozens of witnesses who ran up to the grassy knoll at the time looking for a shooter.
I believe what people do, not what they say.