That mod calls it's cannon ammo with 'magazine' in the name, which means it's getting targeted by your script.
a fix would be to exclude ammo that has 'canon' or 'shell' in the name, or to make sure that the ammo belongs to the bullet category. I can't figure out how to do the latter ATM though, since it doesn't allow me to look into ammo_type for some reason.

I figured our what was the issue there. Adding an extra condition on the magazine change to find the keyword 'bullet' in the ammo type should reduce the heavy handedness of your script.