25x184mm NATO Ammo Container
From Space Engineers Wiki
Ammunition for the Gatling Gun and Gatling Turret. Each box holds enough ammo for approximately 2-3 seconds of constant firing.
As of December 2014, when you pull the trigger with this ammo, the gun will sometimes instantly burn half of the total ammo currently in the gun's inventory. It is best to load one box at a time into turrets until this long standing bug is addressed.