These drums are made of mild steel.
Plain (Internal Surface): Plain here refers to the internal finish of the drum. Unlike epoxy or lacquer-coated drums (which have a protective coating inside), a plain drum has just the bare steel (or a very minimal treatment).
Capacity & Build: Common capacity for such drums is 200 litres (for industrial MS barrels).
The steel sheet thickness used can vary (e.g., 0.8 mm / 1.0 mm / 1.2 mm / 1.6 mm) depending on manufacturer and use-case.
They may have two bungs (holes) or other openings for filling/emptying.
Uses: MS Plain Drums are used for storing and transporting a wide variety of materials: chemicals, solvents, oils, semi-solids, etc.
Because the interior is plain (uncoated), they may not be suitable for very corrosive or reactive chemicals unless additional liners or coatings are used.
Variants: Apart from plain, MS drums can also have epoxy-coated insides for better chemical resistance. They can be open-top (removable lid) or close-top (sealed / fixed head) depending on design.