Another thing to do would be to simply observe at your field as to which barrels work with which paints being shot there, and if it's a barrel commonly ordered in specific bore sizes (not just all medium bore), ask those who use them which bore size it is. Paint size is, however, an average and occasionally you will find days where everyone is ticked off because their paint of choice that is usually a specific size comes in smaller or larger than usual and throws them all off. However, you can feel safe to trust local info for a fair majority of the time because they have already paid the money a few times over to get an effective barrel for what they have available, so you can enjoy the benefit of their research.
(Restated for those short on time: Do as the locals do.)