The size and weight of a barrel varied and was often dependent on whether the contents were liquid or solid. In the United States, statutes generally regulated that a barrel should contain 28 to 31½ gallons. (“Barrel,” in American Dictionary [1828].)
An American Dictionary of the English Language: Intended to Exhibit, I. the Origin, Affinities and Primary Signification of English Words, as far as They Have Been Ascertained. . . . Edited by Noah Webster. New York: S. Converse, 1828.