Barrel (bbl) equals US Barrel and is a recognised unit of volume of US origin and primarily used in the Oil industry, as a unit of volume it is usually referenced to 60*Fahreneit equal to 42 US gallons which is equal 34.9723 Imp gallons or 158.9873 litres both referenced to 15*C
For practical purposes the above are 'accountancy' terms and there is no recognised container for a 'Barrel'
The physical Oil Drum (in some areas incorrectly referred to as a barrel) is a common means of storing/delivering liquid products and typically holds 44 Imperial gallons at 15*C equal to 200 litres at 15*C or 52.804 US gallons at 60*F
PZU - Out of Africa (Retired)