What about the abbreviations "Mbbl" and "MMbbl" that are often found in foreign analytical materials, forecasts, or reports? Behind the single "M" is not the usual SI prefix "mega-", meaning million, but the abbreviated Latin designation for thousand "mille", while the double "M" means million barrels. However, outside the oil industry, life goes on as usual, and Mbbl denotes exactly what it should: megabarrel or million barrels.

Another possible source of this seemingly extra "b" in BBL is related to the tradition of doubling the consonant in abbreviations denoting the plural, or to distinguish it from another North American unit of measurement "bale", which has the abbreviation bl.