I mean :
Pressing the Reload key reload the weapon in the hand of the character, if he have other mags.
Keep pressing the Reload key, the charater take out his mag from the weapon, check how many bullet he have left, and put it back in the gun, while the amount of ammo shows up in the HUD. (In the game, he also yells how many bullets he have left, but I don't think that's very helpful