Barrels are used primarily for the storage of liquids and to age whiskey and wine. Barrels are used in winemaking are typically made of oak, but chestnut and redwood are not unknown. The two major wine uses are fermentation and aging.
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 a wine ages in a barrel, small amounts of oxygen are introduced, a process which is mimicked by 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 prescence 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." 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.
When applied to guns etc, the barrel is the usually metal pipe through which a controlled explosion is released in order to propel a projectile of some kind out of the end at great speed. The first guns were made in a time where metallurgy was not quite what it is today, hence the pipe needed to be braced periodically along its length, producing an appearance somewhat reminiscent of a barrel.
The production of a brewery is also measured in barrels. [Anyone know how big a barrel is?]