I just got a 5 gallon whiskey barrel myself. How full did you fill the barrel? Was there any head space left in the barrel? It was recommended to me that I should not have any headspace in the barrel. I have about 6.5 gallons in my primary and want to make sure after transferring I will have enough considering trub on the bottom of the primary.
Then again if this is a freshly empty barrel and I only have it I for a short period of time, will it even matter with this batch. I say this based on the 3-3-3 method mentioned in this thread.