This is probably because of how vanilla tanks work. Each ammo item always corresponds to one "bullet", while the clipSize corresponds to the full load of the tank. We'd have to look into separating the clipSize from the tank load.
Whoa, I was a few days late with my response and thanks to Fox105iwsp, and you still managed to push a post one minute before me, SupSuper
So... yeah, I did some experiments with Fox105iwsp's way of doing things, I had some interesting effects but no success. I guess we need to wait until (and if) these changes make it to the game, and my minigun tank stays the same for now.
Thanks for your time, both of you.