Using a fast firing gun means you go through stacks of ammo quickly. This means you need to fill up too many inventory slots just with ammo.

I propose ammo crates, either by crafting 2x2 ammo or 3x3, which you then need to craft back into bullets.

OR as an item that works as a 2x2 or 3x3 inventory for only ammo. This way would mean multiple different kinds of ammo could be in the same slot.
Of course you wouldn't be able to stack these.
And maybe you could place them on the ground for decoration.