I see ammo boxes for sale, yet I'm not sure what they're for. They appear in the Miscellaneous section of my inventory. What do I do with them?

Edit: You need to drop and pick them up to get the ammo. It seems buggish.

Think of it this way you can run around with it as a box of ammo or you can drop it and pop it open... How much ammo is in an ammo box?

Depends on the ammo.