You'll have to be more specific about what you are trying to do. Are you trying to finish the beer in the barrel and use it as a serving vessel like a cask? Trying to just add carbonated beer to a barrel and serve back out of it? Do you have a particular barrel in mind? Most barrels are not designed to keep beer carbonated. The wood barrels that used to be used for serving were normally lined with pitch or something else to make them not just watertight but airtight.