A question from the OP about damage.
No. 1; The aircraft is going four or five miles per minute, or say 500 feet per second. The claimed rate of fire of an MG 151 was 750 rpm. That's 11 or 12 rps. The math suggests that a shell hits the ground every 50 feet. Assuming he's using both inboard cannons, thats every 25 feet. Now there's everything wrong with the above math, but you get the idea.
The Germans considered 4 or 5 hits on a B-17 really good results.
They also used explosive shells usually.
I have a good set of pictures of an aircraft that survived getting hit by a cannon shell in the cockpit. In this particular case the target was moving fast and the cannon was stationary. But a cannon shell fired from a fast moving aircraft at a stationary targer will have some of the same effects.
One hit, entry hole about the size of a basketball. Then it blew up inside the aircraft and tore the IP and the pilot up.
Don't do the Snoopy doghouse stitch holes look. If you want to show a soft skin hit, punch one big ragged hole in the sheet metal.