Barrels are used for the storage of liquids, to ferment wine, and to age wine (notably brandy, sherry, and port) and whiskey. Wine is said to be fermented "in barrel," as opposed to a neutral container such as a steel or concrete tank. The barrels used for this can be hundreds or even thousands of gallons.
When applied to firearms, the barrel is the pipe, usually metal, through which a controlled explosion is released in order to propel a projectile out of the end at great speed. The first guns were made in a time where metallurgy was not quite what it is today, so the pipe needed to be braced periodically along its length, producing an appearance somewhat reminiscent of a barrel.
The barrel is also the name of several units. A barrel of crude oil is 42 US gallons, approximately 159 litres. In the UK, a standard beer barrel is 36 UK gallons, about 164 litres. In the US, a standard beer barrel is 31.5 US gallons, about 119 litres.
When a wine or whiskey ages in a barrel, small amounts of oxygen are introduced, a process known as microoxygenation. Oxygen enters a barrel when water or alcohol is lost due to evaporation, a portion known as the "angels' share". In an environment with 100% relative humidity, very little water evaporates and so most of the loss is alcohol, a useful trick if one has a wine with very high proof.
Wine aged in small new oak barrels takes on some of the compounds in the barrel, such as vanillin and wood tannins. The presence of these compounds is dependent on many factors, including the place of origin, how the staves were cut and dried, and degree of "toast" applied during manufacture. After roughly three years, most of a barrel's flavor compounds have been leached out and it is well on its way to becoming "neutral."
The tastes yielded by French and American species of oak are slightly different, with French oak being subtler, while American oak gives stronger aromas [1]. To retain the desired measure of oak influence, a winery will replace a certain percentage of its barrels every year, although this can vary from 5 to 200 percent.
Barrels used for aging are typically made of oak, but chestnut and redwood are not unknown.Barrels for aging