A unique characteristic of most American whiskey types is that they must be aged in new charred oak barrels. Straight bourbon, Tennessee whiskey, and straight rye must be so aged. Since those three types represent such a high percentage of U.S.-made aged spirits, most used American whiskey barrels (hundreds of thousands of them every year) are sent to Scotland, Ireland, Canada, Mexico, Jamaica, and other places where they are used to age virtually every other type of aged spirit.
Heaven Hill is the only major distillery that makes corn whiskey for sale and one of its brands, Mellow Corn, is aged in used barrels.
I assume the barrels Heaven Hill retains are some of the most desirable ones.
The most desirable ones are the youngest, i.e., the barrels that held bourbon for the shortest amount of time. The bourbon that is used for the ready-to-drink market in Australia, where they sell bourbon and ginger ale, and bourbon and cola pre-mixed, is barely two years old--just old enough to be called 'straight bourbon.'
I imagine those are the barrels they retain for aging Mellow Corn.
Mellow Corn is a very small but growing brand. Heaven Hill's principal use for used barrels is for aging their brandies, primarily Christian Brothers. In addition to the youth of the barrels, there is the fact that both the brandies and the bourbon for the ready-to-drinks are aged at Bernheim in Louisville, whereas the rest of Heaven Hill's bourbons are aged in Bardstown and vicinity. I don't know for sure but I suspect Mellow Corn is aged at Bernheim too.
The masonry warehouses at Bernheim are mostly empty, so anything they can age there and not ship to Bardstown they probably do age there. Parker and Craig Beam don't like the way the masonry warehouses age so with the exception of the RTD stuff and some contract production, they don't age any bourbon there.
Most U.S.-made brandy is aged in used bourbon barrels. In addition to Christian Brothers, Heaven Hill also makes Coronet. Korbel brandy is aged in used Jack Daniel's barrels, part of what Brown-Forman pays for the rights to market Korbel Sparkling Wine. Paul Masson Brandy is still aged and bottled at the Barton 1792 Distillery on behalf of the distillery's previous owner, Constellation Brands.
The best-selling U.S.-made brandy is E&J, which is Gallo. They use their own used wine barrels.