Weapon skills

Hello all i dont know if this is the right section to post this but this is only question.

So i want to make skill system based on that how much bullets have been shot lets say for example if player shoots 100 bullets incrase hes skill level by 1 but if he shoots 100 bullets in amunations shoting range incrase skill by 2 o know how to do almoust everything for this except how to count how many bullet have been shoot.

Thanks for advice :)