Using Whiskey Barrels

Has anyone used whiskey barrels for fermentation/aging? I am deciding whether to use the barrel as a secondary or just use it to age a couple weeks after the secondary fermentation but before I keg it? Any suggestions? I am brewing a lightly smoked dark ale. If I use the barrel as a secondary fermenter, I shouldn't have to purge any air since it will have an airlock. But if I just age it, there will be a little air still left in the barrel that I would have to purge and not sure how. Thanks for any help.