The contents of some barrels can vary greatly depending on the purpose of the barrel. For example, a barrel may contain OIL (3 letters: OIL), ammunition (4 letters: AMMO), or SHOT (4 letters: SHOT). OIL is a liquid hydrocarbon that is used for a variety of purposes, such as fuel, lubrication, and heating. Ammunition is a type of explosive material used in firearms and other weapons. SHOT is a type of small metal pellets used in shotguns and other weapons.