In Borderlands 2, carrying enough ammo for your favorite and most powerful weapons can sometimes be the difference between life and death.
Luckily you can increase your ammo capacity with upgrades from the Black Market.
You unlock ammo capacity upgrades for each weapon group separately, and they’re only purchasable with Eridium.
You’ll find the Black Market on the southeastern side of Sanctuary, outside of Moxxi’s place.
The ammo upgrades are called SDU’s.
The first ammo upgrade for each weapon group costs 4 Eridium, and increases by 4 Eridium every time, so it can be a little pricey, but it’s worth every penny.