They work fine for me, I made them.
It sounds like a lot of things are missing, or that you aren't pressing the right button to fire them. Are you sure you're pressing whatever button you've assigned to fire guns only or fire both guns and cannons? If you're only pressing the fire cannons only, only the 20mms will fire. I can't think of how you could have installed it at all and have absolutely nothing happening when you try to fire the 303s.