Looking back through the Text Book of Small Arms - which has hundreds of pages of ballistics analysis - and other contemporary sources, its clear they fired of millions of rounds just in testing and development. IIRC (brain fade) one book even has spark photographs (early high-speed photography) of barrel behaviour during the firing sequence. I guess they did a pretty good job of identifying barrel harmonic points through inspired trial-and-error. Its interesting that some No1 rifles can be so sensitive to tiny adjustments of the inner band screw & spring - clearly the inner band does indeed sit on crucial location on the barrel.