Organize the cartridges into boxes of 20, or whatever suits their load specifics.
For maximum discomfort, carry the boxes in a metal ammo box by the handle. Or for least efficiency, juggle 200 loose rounds in your fingers. Those are your starting points, so choose from there. (When hunting or hiking I distribute the weight and mass around my pockets and packs so there isn't a single large counterbalance on my hips and torso.)