Storage Deck Upgrade

A Storage Device Upgrade (or SDU) is a certain item that you can purchase, to increase the ammo carrying capacity of the respective weapon type. How much a SDU increases the capacity depends on its value. The further it expands your ammo limit, the more expensive it is (also shown by the rarity color). The only SDU you can't purchase is the Backpack SDU, which you can only obtain, if you do the Claptrap Resque quests.