when changing items such as carriers or weapons, your current item disappears along with all of its ammo and or contents, ie:

you find a carrier on the ground, you pick it up, your rangemaster belt disappears, it is not on the ground, your 3 spare mags contained in it are gone you are now left with just the mag in your gun.

this happens to weapons, packs, carriers, and fatigues, whether it be on the ground or in a container, or on a body