Once American whiskey distillers are finished with a barrel they’re sent around the globe for more boozy use.
This article by the Daily Beast serves as a great reminder that bourbon barrels are the gift that keeps on giving. While bourbon itself must be barreled in new barrels, other spirits from scotch to tequila, rum to beer borrow both flavors and even alcohol from the used barrels to provide rich and unique tastes.
Source: The Nine Lives of One Bourbon Barrel – The Daily Beast