Increase the max ammo capacity on your guns, the size of your backpack, and even the depth of your player bank.
Borderlands 3 has millions of guns and weapon choices to pick from, but the amount of ammo available use for those gun stays the same.
However if you have enough money, you can upgrade the maximum amount of ammo you can carry for each weapon type in game and the maximum carry limit.
Head to sanctuary and visit Marcus Munitions to begin upgrading your ammo and carry limit.
If you want a guide here is a hand table made by a Reddit user.
Each upgrade gets progressively more expensive, requiring you to play the game on Mayhem Mode 3 to earn enough money to purchase the upgrades. Mayhem mode is available after you beat the main story.
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