It does this: Top muzzle is used to fire small projectiles at fast rate of fire. If they were fired from the bottom muzzle, they would be very inaccurate. The bottom muzzle is used for
a) big explosive projectile
b) flamethrower attack (this was the initial idea)
c) shotgun mode that combines multiple smaller projectiles..
but just one of them... other two will be made into another weapon(s). The top muzzle would be too small for this alternative attack. And so that's why there are 2 muzzles! Hoorraah! I solved my problem by rambling something about it

Oh and here's the model, you probably wanted to see it more than read my ramblings..
