That is a wonderful argument.
Because your cousin Jesse V said it's impossible.
Why would you even care if your cousin Jesse believes, I don't?
Below is some footage from Tom Alyea. This is filmed inside the TSBD after the shooting. In the 1st part of the film, you will see an FBI agent looking out a window. As he backs up out of the window he looks surprised then changes his direction and walks toward the camera filming him. What is the agent carrying in his left hand?
Pete, there's one thing that we can agree on: The very-low definition of movie cameras (circa 1963) used by news organizations causes great frustration when viewing archival film footage exposed in them . If HD Video Cameras were available in 1963, we could now identify with accuracy details that would eliminate silly theories. Ergo, the exact type of rifle might be discerned.
The rifle carried by the man is not a Mauser. I cannot see a bolt-action, so it might be a M1 Garand? It's probably a rifle carried as an attack/defense weapon.
Incidentally, how do you know the man is an FBI agent?