In resident evil 5 you can manage your inventory during animations like melee attacks or climbing up ladders. What you saw was probably the player reloading their gun by going in their inventory and combining ammo into their gun. That means you never have to go through a reload animation, saving you time in mercenaries, and saving you cash by not upgrading reload speed in the campaign. Such a cool feature.

Edit: My question is why did I get downvoted?