Unfortunately, there are several units called barrel; the summary of the Wikipeda article says (emphasis added): A barrel is one of several units of volume, with dry barrels, fluid barrels (UK beer barrel, U.S. beer barrel), oil barrel, etc. The volume of some barrel units is double others, with various volumes in the range of about 100-200 litres (22-44 imp gal; 26-53 USgal).
For more details: http://en.wikipedia.org/wiki/Barrel_(unit)
This is one important reasons for using the metric system - standardization.
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