Ammo Boxes are an enviromental object found in Batman: Arkham Origins Multiplayer, used for resuppliment. There is one per map.
They appear a grey metal boxes filled to the brim with cartridges, magazines and bandoliers, with shotgun shells lying around it. In Detective vision and in Enchanced Vision, they appear orange and light-blue, respectively.
On the HUD display, there is a special marker that shows how far away the map's ammo box is and in what direction it is.
To use an ammo box, the player needs to walk up to it and press the "use" button. The player will resupply ammo at a rate of 1 magazine per second until the player is at full ammo.
Staying near an Ammo Box will make running out of ammo less of a problem. In addiditon, the chance is that an enemy might show up to resupply himself, allowing you to ambush him as starts using the box. Be careful though, camping around an Ammo box is not recommended for longer periods since enemies can find it on the HUD display.
As a hero, you can lay down Explosive gel on an Ammo box to knock out anyone attempting to resupply their ammo, though this is not the most effective way of using the gel.