When touring distilleries last weekend, Heaven Hill, Makers Mark, Wild Turkey, and Woodford Reserve all claimed to be selling used barrels to age this beer. I can't imagine that the brewery needs a high quantity of barrels, and I would think they would want to reduce flavor variation among the different bourbons that have previously occupied those barrels.
The only thing I can assume is that they use multiple bourbon-barrels, and then mix for consistancy. Can anyone verify that this is the case? Does anyone know what other distilleries might be involved?