John,
The rifle would be pointed on a downward angle causing the muzzle flash to be lower than the user. Also, you would probably have to adjust the location of the muzzle flash for the angle where Mr. Bell filmed from.
If it is not a muzzle flash, what do you suppose it is? That flash does not show any signs of a burnt frame in the film. A burnt frame at the same exact place on the frame where there are people with what seems to be a rifle would be too coincidental in my view.
Just thinking out loud:
1. I doubt an assassin would hang outside a window, but would rather seek concealment
2. The flash is too low to be associated with the people hanging out the windows on the floor above
3. The flash seems to come from a window on the 2nd floor
4. The Bell film seems to show the windows on the 2nd floor as being closed
5. Wouldn't a shot from the 2nd floor shatter a closed window ?
All that being said, I have NO idea what the flash is...and yes, I do find it very intriguing...
Good catch !