Realistically, you can't really feel if a mag is "heavy" or "light" without basically removing the mag to hold. At which point you're dead because you just unloaded your gun in combat for no reason other than to check if you have ammo. Which you now don't.
That's how it is in RO2, you press a button and an animation plays of him pulling the bolt back slightly or pulling the mag out. Then you get a vague message of how much ammo is left. IMO it works well because it takes a bit of time to do and you obviously can't use your gun while doing it. So you have to be out of combat and in a lull during the fighting before you can do it, otherwise you get yourself killed.
Right now in RO2 it might be a bit fast and maybe even too specific (it tells you if it's empty/light/heavy/full). But I still think its a good way of finding out if you have rounds left to fight with before moving on in a firefight.
This is all assuming its even possible to do this in cryengine.