I think that powder burst is from Crooks aiming the weapon and firing towards the crowd. The gun’s ejector or end of the barrel has to be at least 5 inches or more from the metal roof. You have to take in consideration of how high the wind was blowing when he fired. The wind could have caused the powder to be forced downward.
Hopefully a firearms expert will see our posts and weigh-in.
I think most if not all FReepers DO agree that the SS DELIBERATELY IGNORED THE SECURITY PERIMETERS FOR THIS RALLY. NO QUESTION ABOUT THAT, WHATSOEVER.