Definitions of barrel:

• noun:   a cylindrical container that holds liquids
• noun:   a tube through which a bullet travels when a gun is fired
• noun:   any of various units of capacity

  Example: "A barrel of beer is 31 gallons and a barrel of oil is 42 gallons"

• noun:   the quantity that a barrel (of any size) will hold
• noun:   a bulging cylindrical shape; hollow with flat ends
• verb:   put in barrels