This is actually something I've always wondered, seeing as how I don't own a gun, in real life when you reload, in most ammo clip style guns you have to reload the whole clip right? So you should only reload when it is completely empty otherwise you are wasting ammo clips that are still mostly full right? (unless it's a revolver or something where you can load the bullets individually obviously).
I think in games it might make it interesting that if you reload, instead of ammo boxes, you find ammo clips. So for example if you have fired 3 shots of out 12 on a clip, it has 9 left. If you reload it just switches to a new clip in your inventory and puts the 9/12 clip back.
Might make it annoying but more realistic.
Or there could be some thing where you refill the bullets in the clips individually but it takes longer than a quick clip reload.
Ignore this is if I am completely wrong about this clip business. Like I said I know nothing about guns.