I would like to suggest the following: (i know it’s been out there before.)
Effectively these items would take up 1 Slot in your inventory each. This is a simplified version of and includes the “Magic” of video games. But does not allow people to carry 500 rounds that is quickly loadable into a weapon.
Loose: up to 25 rounds for 9mm to 10 rounds for 556 Ammo or Shotgun
Clip: Weapon Specific = 1 reload (Shotgun exempt) Weapons would only be loadable from this item
Box: 75 rounds for 9mm, 50 Rounds for 556 Ammo, 25 for Shotgun Shells
Can: 250 Rounds for 9mm, 125 for 556 Ammo, 100 for Shotgun Shells (additionally Carrying a can would impose a 1% speed reduction)
Additional note: these items should not cost additional materials, just because it would really be annoying. Unless they were made permanent items that could be “unloaded” by manual commands.