Wow just found this picture of a firing Leopold the smoke coming from the barrel looks amazing.

Cool picture

That "flower" pattern might be caused by the rifling of the barrel. There is a short period when the rifling bands on the projectile has left the barrel but the rear end is still blocking most of the barrel so some of the combustion gas (smoke) is pressed out via the rifling grooves.

/ Robin

"The main barrel of the K5 is 283 mm (11.1 in) in calibre (caliber), and is rifled with twelve 7 mm (0.28 in) grooves. These were originally 10 mm (0.39 in) deep, but were shallowed to rectify cracking problems."

From Wikipedia:
https://en.wikipedia.org/wiki/Krupp_K5