Slug Racks are used to carry large amounts of Slugs over long distances. A single slug rack can carry about 20 slugs per barrel. There are 2 main types of slug barrels, the standard Slug Barrels which can come in different sizes and Torpedo Shells. It is unknown if they also come with racks.
Slugs are usually contained in cylinders called Barrels or Shells made of clear blue plastic and metal end caps.
Ghoul Racks have only a different colour scheme, using clear red plastic and black ends. It also appears that the tube entrance can be closed so the ghouls can't escape.