But why is a barrel only 42 gallons and not 55? Because that’s how big whisky barrels were in PA in the 1800s when oil was first shipped.
I found this for you...
HOW BIG IS A BARREL?
A confusing unit of measure is a barrel. A barrel’s capacity is determined often by who uses the term, or what it contains.
For example:
1 barrel (bbl) of petroleum or related products = 42 gallons.
1 barrel of Portland cement is 376 pounds.
1 barrel of flour - 196 pounds.
1 barrel of pork or fish - 200 pounds.
1 barrel of (US) dry measure is 3.29122 bushels or 4.2104 cubic feet.
A barrel may be called a “drum”, but a drum usually holds 55 gallons!
1 barrel going over Niagara Falls was big enough to carry a man.
http://www.eia.doe.gov/kids/energyfacts/sources/non-renewable/barrel.html
Another interesting page...
This measurement originated in the early Pennsylvania oil fields, and permitted both British and American merchants to refer to the same unit, based on the old English wine measure, the tierce.
http://en.wikipedia.org/wiki/Barrel_(storage)