Barrel. When used as a unit of measure, the quantity equal to 31 U.S. gallons. When used as a container, a consumer package or keg containing a quantity of beer listed in 25.156, or other size authorized by the appropriate TTB officer.
27 CFR § 25.11
When used in this part, where not otherwise distinctly expressed or manifestly incompatible with the intent thereof, terms have the meanings given in this section.