First, here is the gif that Robin put up. Although a muzzle flash occurs simultaneous with the sound of the blast, notice that there is no reaction from Oswald, Graves, Leavelle, or anyone else at the instant of the flash.
Here is the alleged muzzle flash as a still:
Unger failed to identify the source of this film, and I don't know who took it. However, it is the very same one used in the newsreel that we saw yesterday. So, we can check for the muzzle flash there. It doesn't occur.
Here they are together in a collage, with and without muzzle flash.
Here's the link to the newsreel video. Start watching at 38 seconds.
There is no muzzle flash, and it is the exact same film.
It means that somebody added that phony muzzle flash to the film that Unger put up.
And don't you think that if a film showing the muzzle flash existed for 53 years that people would have been showing it and viewing it and talking about it for 53 years? How could the shot with the muzzle flash not have become the iconic image from the shooting?
And watch it again. Do you see any recoil? I sure don't.
And, he was firing with just one hand.