I think an expert should weigh in but that neatly formed powder burst to the right of the blood pool surely didn’t come from anyone else but HIS weapon. The DIRECTION of the burst appears to be TOWARDS HIS HEAD and not AWAY FROM HIM, too.
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.