Alan,  I'm guessing that the barrel length rule was a matter of the barrel stiffness and the powder charge burn rate vrs. bore volume.   
I think that many of our ideas on "proper" barrel length are conditioned by tradition set when longer barrels were needed to burn and contain the expanding gas from older slower burning powders.  also barrel balance, harmonics and sight radius (for iron sights of course) are factored in. 
you'll note that most of the modern single-shot benchrest guns are using the shorter stiffer barrels and smaller charges of faster modern ball powder