Looking at the barrel profile (wallthickness) it's easy to get a visuallization of the pressure decay as the payload moves down the barrel. Match up that graph Jack posted and you'll see a barrel profile, even a 100 yr old one, matches up fairly nicely. Makers have matched the barrel profile to the pressure, probably thru trial and error (blowing up barrels) over early years of development. Likely, proof houses had something to do with all this, maybe not. I dunno. But barrels have that taper in the places they have it for a reason.