Here is a demonstration of the Colt Cobra (Ruby's pistol) being fired. It doesn't have much muzzle flash that we can see from this angle, but it has a whole heck of a lot of smoke puff- clouds of it, which you could see from any angle.
So now, watching the shooting of Oswald by "Ruby". I'm putting up this one because the standard ones are all spliced and diced, and they're just showing you what they want you to see; what they will allow you to see. But in this one, it's continuous and uninterrupted. You see "Ruby" go up to Oswald and shoot him, and you see Oswald react and go down, but there is no muzzle flash and not a bit of smoke.
Here Oswald is already reacting to be shot. His grimacing in pain, and he's starting to sink. So, the shot has just gone off. How come no smoke?