bbl vs firkin

bbl

noun
  • Numbers of barrels (casks) transported or sold, or the volume that they represent. 

  • Numbers of venturis in carburetion. 

  • Volumes of crude oil pumped, stored, or sold (in bulk). 

firkin

noun
  • A varying measure of capacity, usually being a quarter of a barrel; specifically, a measure equal to nine imperial gallons. 

  • A small wooden vessel or cask of indeterminate size, used for butter, lard, etc. 

  • A weight measure for butter, equalling 56 pounds. 

How often have the words bbl and firkin occurred in a corpus of books? (source: Google Ngram Viewer )