500 bullets stacks

The existing solution for saving space is to learn to bulk craft boxes of ammunition. Each box converts to 100 rounds when opened, and you can have a stack of many boxes. It's also more efficient to craft, requiring just 80% of the ingredients it would take to make individual rounds. I believe the recipes to craft boxes of ammunition are all unlocked by reading the corresponding set of books, like Pistol Pete for boxes of 9 mm bullets.