Whats not possible is to have more than one weapon on a plane that the user can control properly, i.e. the gun would not move relative to the planes pitch and bank.
Take a ball turret on a Flying Fortress, it would not pitch or bank with the plane, and a human gunner could point the gun into the interior of the aircraft. Collision detection unfortunatly does not work with this method.
For AI guns it's is possible, using DoWatch and the pitch\bank functions we developed for MCar, you could make the AI turret look like it's conforming to the pitch and bank of the vehicle. But all of this would require a ton of scripting, for what would essentialy be, laggy eye candy.
I would go with m21man's suggestion, at a push you could make it look like it's firing from multiple guns for cut scenes e.t.c