The problem with mine is that it is a tank turret. You see where it says
numMountPoints = 4;
mountThread[0] = "sit";
(That should be 1 by the way.) I think that might be the problem. I don't know why mount3 is not shooting. It might be that I didn't add it there but even when I did, it still did not work. Also, the mech is a one person vehicle. I can get non turret vehicles to shoot multiple projectiles fine.