When using the 'View Contents' action on an ammo box, you can not move items (I.e. stacks of ammo) into the 'Vicinity.' I.e. say you want to remove a stack of ammo from the box, you use 'View Contents' so as to select the ammo but when you attempt to drag-and-drop the stack out of the box, but in to the Vicinity pane, it will not work. You have to move the ammo into your inventory, then cancel 'View Contents' either by pressing 'Tab' or 'Esc', then re-access your inventory and drag the ammo into the Vicinity pane to dispose of it.