They seem to fire horizontally from the plane depending on the direction it faces. Tested with a custom plane modeled off of the Fokker, and with the P-51 Mustang.

From what I've seen it appears that there's a messed up or forgotten radian - degree calculation somewhere. Only taken a quick look though.

EDIT: Actually scratch that, looks more like the bullets are shooting at 360-plane's angle

Sounds good, thanks for fixing this. Looking forward to seeing a pull request :)

I've managed to fix the bullet rotation issue, but it needs some cleaning, at the moment I'm creating my own matrices rather than using pre-built ones.