I read the original question as to the origin of "Bbl" as the abbreviation for barrel or barrels. Here's what I foung by entering Bbl on Google: The "b" may have been doubled originally to indicate the plural (1 bl, 2 bbl), or possibly it was doubled to eliminate any confusion with bl as a symbol for the bale (see above). Note: Some web sites are claiming that "bbl" originated as a symbol for "blue barrels" delivered by Standard Oil in its early days; this is almost certainly incorrect because there are citations for the symbol at least as early as the late 1700s, long before Standard Oil was founded.[3]
From Wikipedia. If the question was why are they called barrels. that's because the first ones for guns were made by "barrel", as in cask, makers.
