I never saw a flash from a main gun like that on the firing range, even at night if I am not badly snakebitten.
Maybe the film or digital camera responds over a wider light spectrum?
Sicily is the largest DZ on Fort Bragg and I bet a C-17 couldn't get a full load of paratroopers out over the smaller ones. I've seen a drop like that from the ground; you think they will never stop exiting that aircraft.
Sicily Drop Zone