Steve, I stand corrected and better informed, thank you!
A barrel is a hollow cylindrical container, traditionally made of vertical wooden staves and bound by wooden or metal hoops. Barrels often have a convex shape, bulging at the middle. This constant bulge makes it easier to roll a well-built wooden barrel on its side, changing directions with little friction. The barrel became a standard size of measure referring to a set capacity or weight of a given commodity.
A beer barrel had originally a capacity of 36 US gallons (140 L)
An ale barrel has capacity of 32 US gallons (120 L).
Basic Wine barrel is 31.5 US gallons (119 L), Bordeaux type 59.4 US gallons (225 L), Cognac type" 79.2 US gallons (300 L).
Oil has not actually been shipped in barrels since the introduction of oil tankers, but the 42-US- gallon size barrel is still used as a unit for measurement, pricing, and in tax and regulatory codes.
Barrel of crude oil or other petroleum product is 42 US gallons (34.9723 imp gal; 158.9873 L).
Barrels for chemicals and food is 55 US gallons (46 imp gal; 208 L).
A Barrel is also an important part of a gun!