After seeing the dragon getting blasted to pieces like that, I had to look up Kanno Naoshi. He was flying a Kawanishi N1K2-J "Shiden-Kai" during his last battle. While I couldn't confirm the armament on his variant, it seems that 20 mm autocannons were frequently installed on that line of aircraft. 20 mm rounds are BIG. Some are explosive as well. So, yeah, the scene seems legit. Poor dragon!